Soft brake pedal
Thread Starter
Instructor
Joined: Feb 2001
Posts: 224
Likes: 41
From: Catawba, N.C. USA
Soft brake pedal
I have a 1976 T-top. I replaced the rear brake calipers and brake lines. I bled the system three ways. First with a vacuum pump (Mityvac); then had a friend pump the brake pedal; still not satisfied I got a pressure bleeder and pressurized the system. I believe I have all the air out, however I ran the car and the brake pedal is soft and doesn't stop the car until the pedal is on the floor, but then will stop the car. It doesn't feel right. I ran 2 and 1/2 quarts through the system until it ran clear with no bubbles. Do I still have air in the system? Could it be the brake booster? Is there a pedal adjustment I should be doing? This is really frustrating. Any help is greatly appreciated.
Pro
Joined: Dec 2005
Posts: 720
Likes: 6
From: Oklahoma City Okla
Soft Pedal
I feel your pain 
I have a 78 that has the same problem. I have put new calipers, master cylinder, bleed the lines using the mityvac, pedal bleed, and gravity. The pedal is still soft and is driving me crazy 
I am using synthetic fluid and I would bet that is the problem. The rotors are running out in spec and if there is air in the lines (which I think there is) I can't imagine how there could be as I also ran two quarts through the damn system. I am going to try to flush the system and replace the synthetic with either Castrol LAM or NAPA DOT 4. I have heard you must disassemble the entire system to properly flush the system but I may just try to flush the damn thing using the pressure bleeder. I am almost at the point of giving up 
and taking it to a stealer ship or a hot rod shop that specializes in vettes. Good luck and if you find a fix please let me know. 
I have a 78 that has the same problem. I have put new calipers, master cylinder, bleed the lines using the mityvac, pedal bleed, and gravity. The pedal is still soft and is driving me crazy I am using synthetic fluid and I would bet that is the problem. The rotors are running out in spec and if there is air in the lines (which I think there is) I can't imagine how there could be as I also ran two quarts through the damn system. I am going to try to flush the system and replace the synthetic with either Castrol LAM or NAPA DOT 4. I have heard you must disassemble the entire system to properly flush the system but I may just try to flush the damn thing using the pressure bleeder. I am almost at the point of giving up and taking it to a stealer ship or a hot rod shop that specializes in vettes. Good luck and if you find a fix please let me know.
Melting Slicks
Joined: Mar 2007
Posts: 2,100
Likes: 12
From: Frederick Maryland
If you want to change back from synthetic brake fluid...make DAMN sure you flush it completely and make sure it's completely purged. Synthetic and DOT3 or 4 fluids do not mix under any circumstances and will cause you problems...soft pedal is the least of them but probably the most common.
Most, if not all, performance car makers specifically prohibit DOT 5 fluid in their cars...especially if they have ABS. Despite DOT 5's advantages in not absorbing moisture, it will aerate in hard maneuvering and cause pedal problems. Mix DOT 5 with DOT 3 or 4 and you'll have a milky and cloudy mess that can congeal in your brake lines and master cylinder.
There's been reports and some evidence of DOT 5 silicone fluid disabling the hydraulic type of brake light switches in some cars due to non-compatibility.
Leave DOT 5 to special applications like cars in a museum.
If you think your brake booster might be a problem, disconnect the vacuum line from it and see if the engine runs any different. If it does, you have a vacuum leak inside the booster. If your engine runs worse until you plug the vacuum line, the booster is probably OK.
One thing to look for, and this happened to me on my '78, is the caliper pistons. It seems to be not uncommon for this generation brake system, while well engineered, will corrode if not driven regularly, or the brakes pumped regularly if the car sits a lot.
The caliper seals need to be kept lubricated. If not, they can leak slightly...not leak fluid, but will suck air into the system and show no visible problems. This happened to me, and the calipers weren't that old.
Most, if not all, performance car makers specifically prohibit DOT 5 fluid in their cars...especially if they have ABS. Despite DOT 5's advantages in not absorbing moisture, it will aerate in hard maneuvering and cause pedal problems. Mix DOT 5 with DOT 3 or 4 and you'll have a milky and cloudy mess that can congeal in your brake lines and master cylinder.
There's been reports and some evidence of DOT 5 silicone fluid disabling the hydraulic type of brake light switches in some cars due to non-compatibility.
Leave DOT 5 to special applications like cars in a museum.
If you think your brake booster might be a problem, disconnect the vacuum line from it and see if the engine runs any different. If it does, you have a vacuum leak inside the booster. If your engine runs worse until you plug the vacuum line, the booster is probably OK.
One thing to look for, and this happened to me on my '78, is the caliper pistons. It seems to be not uncommon for this generation brake system, while well engineered, will corrode if not driven regularly, or the brakes pumped regularly if the car sits a lot.
The caliper seals need to be kept lubricated. If not, they can leak slightly...not leak fluid, but will suck air into the system and show no visible problems. This happened to me, and the calipers weren't that old.
Pro
Joined: Dec 2005
Posts: 720
Likes: 6
From: Oklahoma City Okla
Brakes
I have put new Lone Star calipers on the rear because the bleeder screws were broken. It has Valvoline synthetic fluid now and I want to change to a NAPA DOT 4 or some other DOT 4 fluid with a higher boiling point and less problems with air absorbtion. Maybe I'm just not patient enough while trying to gravity bleed but I'm sick of sub standard braking (soft pedal) What is the best way to completly flush the synthetic fluid out? I know the different fluids don't mix well and can cause major problems. Would the Promotive pressure bleeder work to change/flush the synthetic fluid? Any suggestions would be appreciated.
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
If you want to change back from synthetic brake fluid...make DAMN sure you flush it completely and make sure it's completely purged. Synthetic and DOT3 or 4 fluids do not mix under any circumstances and will cause you problems...soft pedal is the least of them but probably the most common.
Most, if not all, performance car makers specifically prohibit DOT 5 fluid in their cars...especially if they have ABS. Despite DOT 5's advantages in not absorbing moisture, it will aerate in hard maneuvering and cause pedal problems. Mix DOT 5 with DOT 3 or 4 and you'll have a milky and cloudy mess that can congeal in your brake lines and master cylinder.
===> They prohibit DOT5 only because of ABS. ABS pulsing aerates the DOT5 fluid. If it weren't for ABS, I'd bet DOT5 would be in nearly all new cars.
There's been reports and some evidence of DOT 5 silicone fluid disabling the hydraulic type of brake light switches in some cars due to non-compatibility.
===> Only if the brake switch AKA "proportioning valve" is not completely purged. In order to purge it, you must disassemble it.
You cannot "flush" it. There is a chamber in the center of the switch where fluid does not "flow".... it just "sits". You cannot flush this area. I disassemble & rebuild these, so I see the mess first hand.
Leave DOT 5 to special applications like cars in a museum.
===> I have DOT5 in both of my Vette's... C2 & C3. In the C2 since about 1976. That's over 30 years. At 20 years, I drained it, filtered the rotten rubber out of it, and re-used it. I have not experienced a single problem or issue with DOT5 in all that time. DOT5 in the C3 for about 10 years. No problems there either. Both cars braking systems are 100% rust free on the inside. Last year, I drove the C3 as a daily driver from November to June. Brakes performed flawlessly. Nothing wrong with DOT3/4 as long as you regularly flush your system. I am curious what your aversion to DOT5 is?
If you think your brake booster might be a problem, disconnect the vacuum line from it and see if the engine runs any different. If it does, you have a vacuum leak inside the booster. If your engine runs worse until you plug the vacuum line, the booster is probably OK.
One thing to look for, and this happened to me on my '78, is the caliper pistons. It seems to be not uncommon for this generation brake system, while well engineered, will corrode if not driven regularly, or the brakes pumped regularly if the car sits a lot.
===> With DOT3/4 the system absorbs water. It must be bled regularly regardless of how often it is driven.
The caliper seals need to be kept lubricated. If not, they can leak slightly...not leak fluid, but will suck air into the system and show no visible problems. This happened to me, and the calipers weren't that old.
===> Air gets into the system if your rotors are not true. As the rotor spins, the pistons (in contact with the rotors) vibrate in & out causing "air pumping". GM turned the rotors & spindles as an assembly at the factory (minimize rotor runout) for this reason. Also... the leaking is not a matter of lubrication... it's a matter of gravity. As the car sits for a long period, the weight of the piston presses down on the bottom of the factory piston lip-seal and it flattens out at the bottom. This is known as "flat-spotting". When this happens, the seal can no longer contain the brake fluid. This happens with all types of brake fluid if the car sits for a long time. The aftermarket "o-ring" design pistons/seals help avoid this problem.
I have lip-seal GM aluminum pistons in my C2 and flat-spotting happens with that car.
I have SS Brakes Corp solid stainless pistons in my C3 and flat-spotting happens faster in that car due to the added weight of the pistons.
To avoid flat-spotting & leaking, drive the car often, or switch to "o-ringed calipers".
Most, if not all, performance car makers specifically prohibit DOT 5 fluid in their cars...especially if they have ABS. Despite DOT 5's advantages in not absorbing moisture, it will aerate in hard maneuvering and cause pedal problems. Mix DOT 5 with DOT 3 or 4 and you'll have a milky and cloudy mess that can congeal in your brake lines and master cylinder.
===> They prohibit DOT5 only because of ABS. ABS pulsing aerates the DOT5 fluid. If it weren't for ABS, I'd bet DOT5 would be in nearly all new cars.
There's been reports and some evidence of DOT 5 silicone fluid disabling the hydraulic type of brake light switches in some cars due to non-compatibility.
===> Only if the brake switch AKA "proportioning valve" is not completely purged. In order to purge it, you must disassemble it.
You cannot "flush" it. There is a chamber in the center of the switch where fluid does not "flow".... it just "sits". You cannot flush this area. I disassemble & rebuild these, so I see the mess first hand.
Leave DOT 5 to special applications like cars in a museum.
===> I have DOT5 in both of my Vette's... C2 & C3. In the C2 since about 1976. That's over 30 years. At 20 years, I drained it, filtered the rotten rubber out of it, and re-used it. I have not experienced a single problem or issue with DOT5 in all that time. DOT5 in the C3 for about 10 years. No problems there either. Both cars braking systems are 100% rust free on the inside. Last year, I drove the C3 as a daily driver from November to June. Brakes performed flawlessly. Nothing wrong with DOT3/4 as long as you regularly flush your system. I am curious what your aversion to DOT5 is?
If you think your brake booster might be a problem, disconnect the vacuum line from it and see if the engine runs any different. If it does, you have a vacuum leak inside the booster. If your engine runs worse until you plug the vacuum line, the booster is probably OK.
One thing to look for, and this happened to me on my '78, is the caliper pistons. It seems to be not uncommon for this generation brake system, while well engineered, will corrode if not driven regularly, or the brakes pumped regularly if the car sits a lot.
===> With DOT3/4 the system absorbs water. It must be bled regularly regardless of how often it is driven.
The caliper seals need to be kept lubricated. If not, they can leak slightly...not leak fluid, but will suck air into the system and show no visible problems. This happened to me, and the calipers weren't that old.
===> Air gets into the system if your rotors are not true. As the rotor spins, the pistons (in contact with the rotors) vibrate in & out causing "air pumping". GM turned the rotors & spindles as an assembly at the factory (minimize rotor runout) for this reason. Also... the leaking is not a matter of lubrication... it's a matter of gravity. As the car sits for a long period, the weight of the piston presses down on the bottom of the factory piston lip-seal and it flattens out at the bottom. This is known as "flat-spotting". When this happens, the seal can no longer contain the brake fluid. This happens with all types of brake fluid if the car sits for a long time. The aftermarket "o-ring" design pistons/seals help avoid this problem.
I have lip-seal GM aluminum pistons in my C2 and flat-spotting happens with that car.
I have SS Brakes Corp solid stainless pistons in my C3 and flat-spotting happens faster in that car due to the added weight of the pistons.
To avoid flat-spotting & leaking, drive the car often, or switch to "o-ringed calipers".
Melting Slicks
Joined: Nov 2001
Posts: 2,668
Likes: 43
From: Eastern US XX
I have a 1976 T-top. I replaced the rear brake calipers and brake lines. I bled the system three ways. First with a vacuum pump (Mityvac); then had a friend pump the brake pedal; still not satisfied I got a pressure bleeder and pressurized the system. I believe I have all the air out, however I ran the car and the brake pedal is soft and doesn't stop the car until the pedal is on the floor, but then will stop the car. It doesn't feel right. I ran 2 and 1/2 quarts through the system until it ran clear with no bubbles. Do I still have air in the system? Could it be the brake booster? Is there a pedal adjustment I should be doing? This is really frustrating. Any help is greatly appreciated.
A few things to keep in mind (some very basic so don't be insulted if you are well aware of them):
You MUST work on getting the air out and KEEPING it out. I was only successful using a pressure bleeder (MityVac NEVER worked). On the same note, you also need to be SURE there isn't excessive runout on any of the rotors - if there is enough runout, it will cause a caliper to "suck air" from the constant fluttering. Verify this first.
Make sure to follow the CORVETTE bleeding order - different from passenger cars of the same era.
Make sure the distribution valve (may actually be a Proportioning valve on your year) isn't "stuck". This is typically identified by low bleed pressure in either front or rear "system".
[VERY obvious but often missed!] Make sure you have plenty of brake fluid in the M/C at all times - keep your pressure bleeder filled sufficiently.
Hang in there - you'll get it!
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Advanced
Joined: Mar 2007
Posts: 97
Likes: 0
From: Colusa Ca
soft brake
My 77 has the master cylinder mounted at a slight angle. This makes for a great place to trap air. you will need to bleed the M/C. Try this, Have someone help you, if you have bleeders on the m/c it helps but you can remove the line and have someone slowly push down on the brake (dont pump it up) pushing the air and fluid out, while holding the brake to the floor close the bleeder or use your thumb and plug the hole now let the pedal back up. do this until you get good fluid and no air on the last time while holding the pedal down close the bleeder or put the line back on. now you need to bleed your brakes again, use this same method again dont pump it up. pumping up can cause too much preasure and the air will not move. My brother taught me this trick he has used it for years.
I just did caliper kits on all 4 wheels and will be bleeding them using this method.
good luck
Paul
I just did caliper kits on all 4 wheels and will be bleeding them using this method.
good luck
Paul
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
Le Mans Master
Joined: Jan 2007
Posts: 7,947
Likes: 3,315
From: Omaha NE
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I put new front brake lines on mine last year. Had a buddy help me bleed it the old school way like dc502 said & my pedal is rock hard. If the fluid is clear and there's no bubbles, the air is out of the system. Rotor runout & the booster are the next things I would check.
If you have synthetic ditch it.
I've never had to bleed my M/C in 30 years. Unless it got so low with fluid that it sucked air I don't know why you would have too. Lots of cars have the M/C at an angle........
If you have synthetic ditch it.
I've never had to bleed my M/C in 30 years. Unless it got so low with fluid that it sucked air I don't know why you would have too. Lots of cars have the M/C at an angle........
Racer
Joined: Oct 2006
Posts: 293
Likes: 0
From: Colorado Springs Colorado
I went three or four "rounds" with the brakes on my 73 before I got them figured out. When I was done I had replaced all calipers, pads, rubber lines w/ SS lines, old rotors w/ slotted, master cylinder and brake booster. Basically, a new system but that was what the car needed.
Biggest lessons learned:
1. Dont waste time with trying to make the old brake system work. Replace the critical components if they are 10+ years old (that is my rule of thumb). Critical components = master cylinder, brake booster, calipers, rubber lines, etc. Dont let "low mileage" convince you that the system shouldnt need replacing. Even though a car may not have enough miles on it to warrant replacing brake parts, the over all design of these brake systems is rather poor not to mention that lack of use is the single biggest culprit when it comes to faulty vette brakes. For example, Vette Brakes Co is in business because of the notoriously sh!tty calipers GM originally put on these cars.
2. No synthetic, not ever. You simply dont need it w/ the stock system -- synthetic only gains you more problems.
3. If you replace your claipers DO NOT go through Auto Zone or any other generic retail parts dealer. For example, I did this exact thing and I had three of the four claipers fail -- not pretty. Go with the newer design SS o-ring calipers. I got mine from Vette Brakes.
3. Motive pressure bleeder and large "C" clamp; dont go over about 12psi or you will suck air into the system. This bleeder allows for a one-man brake bleeding operation. Very nice...
Cheers,
Crunch
Biggest lessons learned:
1. Dont waste time with trying to make the old brake system work. Replace the critical components if they are 10+ years old (that is my rule of thumb). Critical components = master cylinder, brake booster, calipers, rubber lines, etc. Dont let "low mileage" convince you that the system shouldnt need replacing. Even though a car may not have enough miles on it to warrant replacing brake parts, the over all design of these brake systems is rather poor not to mention that lack of use is the single biggest culprit when it comes to faulty vette brakes. For example, Vette Brakes Co is in business because of the notoriously sh!tty calipers GM originally put on these cars.
2. No synthetic, not ever. You simply dont need it w/ the stock system -- synthetic only gains you more problems.
3. If you replace your claipers DO NOT go through Auto Zone or any other generic retail parts dealer. For example, I did this exact thing and I had three of the four claipers fail -- not pretty. Go with the newer design SS o-ring calipers. I got mine from Vette Brakes.
3. Motive pressure bleeder and large "C" clamp; dont go over about 12psi or you will suck air into the system. This bleeder allows for a one-man brake bleeding operation. Very nice...
Cheers,
Crunch
Former Vendor
Joined: Nov 2005
Posts: 683
Likes: 1
From: Mounds View Minnesota
Yep, bleed bleed bleed!
But if your pedal goes to the floor something IS wrong.
If you can pump it up and it stays there with pressure on the pedal there is still air in the system
If you can pump it up and the pedal drops slowly to the floor your master cyl is leaking internally, most likely.
Another idea to try is to clamp off all the rubber lines and see how the pedal feels. If it's still soft it's not the calipers!
Then unclamp the lines one by one to check each caliper...
Some people seem to like synth, so I wouldn't worry about that.
If it was clogging the pedal would be rock hard, not soft I'd think...
Keith
But if your pedal goes to the floor something IS wrong.
If you can pump it up and it stays there with pressure on the pedal there is still air in the system
If you can pump it up and the pedal drops slowly to the floor your master cyl is leaking internally, most likely.
Another idea to try is to clamp off all the rubber lines and see how the pedal feels. If it's still soft it's not the calipers!
Then unclamp the lines one by one to check each caliper...
Some people seem to like synth, so I wouldn't worry about that.
If it was clogging the pedal would be rock hard, not soft I'd think...
Keith
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
How and why do I bench bleed a master cylinder?
When installing or replacing a master cylinder, it is critical that all air is removed from the master cylinder. This can easily be done by bench bleeding the master cylinder prior to installation. Using the SSBC master cylinder bleeder kit (#0460):
1) Place your master cylinder in a vise by the ears (not body). Make sure it is level.
2) Attach a piece of clear plastic hose to the short end of one of the plastic
nozzles. Do the same to the other hose and nozzle.
3) Clip the plastic bridge to the wall and push the ends of the hose through the holes so they are SUBMERGED in the reservoir on either side of the wall.
4) Press the tapered end of the nozzle FIRMLY into the cylinder port hole with a twisting motion. Repeat this procedure on the other port hole.
5) Fill the reservoir with CLEAN brake fluid recommended by the manufacturer.
6) Using full strokes, push the piston in, then release. Do this until ALL the air bubbles have disappeared from the clear plastic hose.
(CAUTION-MASTER CYLINDER WILL NOT BLEED PROPERLY UNLESS HOSES ARE SUBMERGED IN BRAKE FLUID UNTIL THE BLEEDING PROCESS IS COMPLETED.)
Now mount master cylinder and avoid brake fluid leaking out of front and rear ports during installation.
http://stainlesssteelbrakes.com/tech...er%20cylinder?
Instructor
Joined: Apr 2001
Posts: 153
Likes: 2
From: Belle Mead, N.J., U.S.A.
Success! I followed the advice of several responders and now have a brake pedal. First I checked the brake booster by disconnecting the vacuum line while the engine was running and it caused the engine to run poorly so I presumed the booster was good; then I put the car up on floor jacks once again and used the motive pressure bleeder using a large C-clamp instead of the provided chain system (much better seal), I pressurized to 12 lbs. instead of the recommended 20 lbs. (no mess at all); I started the bleed sequence at the driver side rear as recommended and worked my way around and voila, no air in the system and finally got on the road Thanks for all the good advice.
Drifting
Joined: Sep 2006
Posts: 1,986
Likes: 21
From: Columbus MN
Using the SSBC master cylinder bleeder kit (#0460):
http://stainlesssteelbrakes.com/tech...er%20cylinder?
http://stainlesssteelbrakes.com/tech...er%20cylinder?
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
pm me with your address and i send you the one i just used and then you can mail it back to me, bob