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I've had my '78 Corvette for about 5 years now, about 3 years ago my brakes went out completely. The pedal went straight to the floor and the car never even slowed. I took into a shop that replaced the master cylinder 3 times in about a month and a half, got tired of them not digging deeper into the problem, so took it to another shop that came highly recommended. They claimed to have replaced the master cylinder, the brake booster, the front and real calipers, and the brake pads, plus bleed the lines multiple times. The car didn't even make it the few miles home before the brakes went out again. Called and called, they kept pushing me off and wouldn't take the car back in.
Long story short, about 8-9 months later, finally got the money together (had already spent nearly 4.5k with the previous 2 mechanics) and took it into a new mechanic across town. Very honest and kind, sent me pictures of my break calipers, which were most definitely NOT new or replaced recently. The calipers were also dirty and old, there was a lot of rust. Had to replace calipers, pads, springs, even some hose that were too old. The car drove great, no problems at all, the brakes were a tiny bit soft, but it stopped well, I was finally feeling comfortable with it.
And now we're back. I moved from LA to Las Vegas just a few weeks ago. Had the car auto transported over, no problems at all. The guys were great loaded it up and unloaded it just fine. After the truck leaves, I go to pull it into the driveway and there are no brakes. Nothing, my foot and pedal are all the way to the floor and the car is rolling away. I checked the master cylinder, clean and full. I have no idea what it could be. I can't afford to take it to a new mechanic out here right now and I can't see the last mechanic though his work is still under warranty since he's out of state.
At this point, I'm so fed up. I've done other work on the car myself, honestly I prefer that. So any feedback would be appreciated. I'm looking to start working on the brakes myself, just not sure if bleeding the brakes is really the best option right now, or if there's something else I need to look at first.
Do you have someone to pump the brakes while you open a bleeder and see if fluid comes out? That's where I would start. A screwed up master cylinder, wrong plunger rod, air in the lines, etc could be the problem.
Sorry this nightmare is keeping you from emjoying the car.
Welcome to working on your own car....fixing it yourself is so much more satisfying.
A couple of points I'd like to make....
-Bench bleed the crap out of master cylinder. The design of these tend to trap air bubbles, so you need to tilt it up while slowly working the piston in and out....do it slow or you'll create your own bubbles.
- The bleeding process is RR Inner, RR outer..........LR inner LR outer,..... if you don't do it this way you'll be there for days!
-Then do right front and then left front.
Tap on the calipers with a rubber mallet just before bleeding to dislodge any air bubbles so you can get rid of them.
If you have a helper pumping the brakes, ask them to pump slowly......don't create your own bubbles!!
I agree you could start by bleeding the brakes and see if the pedal hight improves.
With the rear up in the air grab each tire at 6 & 12 o'clock and then at 3 & 9 o'clock and post back how much play you have.
You have to remove each rear tire anyway for access to the outer bleeder.
Note: bleed the inner first then the outer.
Next, check the front wheels for play the same way.
When bleeding have your helper press down slowly just once and hold the pressure as you open and close each bleeder.
If you get good brakes and then after some miles and the pedal goes down again you might need to check the run-out.
I don't see a brake bleed being help at all, that much air doesn't just appear...
If no leaks I'd say it's a master problem most likely. A defective booster would just give you a hard pedal. Of course checking to see if rod has come off pedal would be a good idea since the mechanics you've had working on it should bw cleaning the toilets instead of touching cars..
Sounds like that c3 is trying to kill you.
Mine did this on the first drive (pic). Rear caliper bled out within 5 miles of my house.
I got a rebuilt caliper that didnt fit right. Eventually rebuilt the original caliper myself. Recently noticed my master is leaking onto the booster
I would guess your vette has more than one issue.
Some guesses are:
-warped rotor causes caliper to gulp in air
- master push rod mis adjusted causing intermittent blocked port in master
- bad flare fitting somewhere
- pedal assembly pushing master at a strange angle
I would recomend bleeding your car. I really have good sucess with gravity bleed.
Jack the car up. Remove wheels and master resevoir cap. Open a bleeder, any bleeder in any order. Watch bubbled burp out then steady slow trickle come
out of bleeder screw. Close the bleeder. Do the next bleeder.
Mine always has air in the rears not the fronts. When I think of it and I am doing wheels off work I open a rear bleeder and burp out a couple mini bubles. No pedal pressing just gravity bleed.
Warped rotor causing caliper to gulp air?
The system is a closed system, any air is a result of a blown seal/leak somewhere.
A soft pedal is usually air in system, a sudden complete loss of brakes with nothing leaking is not air in the system.
Excessively worn bearings or a warped rotor will introduce air into the brake calipers.
You will not see a leak but air will come out as you bleed the brakes.
You picked up air in the system somewhere. Look closely for any 'fuzz' anywhere on the calipers or under master cylinder on booster. If any fluid can get out...air can get in. Pull master cylinder away from booster 9leave lines in place) and inspect it closely for any signs of wetness. ANY signs.
With 4 pistons on each wheel...Vette's have a lot of places to pull air into system.
You can bleed it and get a good pedal again...but sooner or later it will do it again unless you find the leak. It won't necessarily show as a real wet leak...but just dusty fuzz.
hate to hear you've been ripped off by other shops. Vette's have some unique quirks with brakes...but it's still just hydraulics. If it's air tight it will never give an issue and have a good pedal.
Have the rubber 'jumper' hoses( between hard brake lines and calipers) been replaced? If they are 'original' to the car, they could be completely deteriorated internally and blocking most of the braking force. I certainly agree that bleeding the master cylinder (you can do that in the car, too...by just pulling the brake lines off the M/C and inserting the bleeder hoses required). is a good idea. You also need to check for proper actuator pin length [from booster to M/C). Another possibility [maybe low likelihood] is that the brake distribution block has something going on with it internally which prevents transmission of brake fluid pressure passing thru it.
There are a couple of things the average mechanic is not aware of when working on C3 brakes.
First, like someone else mention, the distance between the back MC piston and the boosters rod is critical. Often the wrong MC is installed because of the wrong year or one MC is for PB and one unit is for manual brakes. They are different in the back, at the mounting casting and the piston depth inside the casting. Perhaps the booster rod was changed also. Who knows?
Second: if a mechanic did indeed bench bleed the MC in a vise and went too far in the casting with a screwdriver, THAT will damage the piston seals. (don't ask how I know) Clearly printed on the MC box is a label: Do Not Push phillips screwdriver Into Bore More Than 1 & 3/8 inch or you will damage unit and void the warranty.
I think that is why your mechanic kept changing out the MC.
Click on my profile and have a look at Bench Bleeding the MC.
I'm betting that ...NOBODY CHECKED ROTOR RUN-OUT and the rotors are out of spec.(wobbling) The MAX spec is .005" I recommend shimming the rotors until they read .001"
When the rotors wobble they 'pump air' into the brake caliper bores....hence no pedal.
PLEASEBUY THE CHEVY CORVETTE SERVICE MANUAL (FOR YOUR YEAR)....BEST $57 YOU EVER SPENT!
The tank is excellent with the gauge. Hose could use a shut-off valve.
The MC lid adaptor is so-so. You can't see inside the MC.
The Motive clamping system, is bird-feeder junk.
Welcome to working on your own car....fixing it yourself is so much more satisfying.
-Bench bleed the crap out of master cylinder. The design of these tend to trap air bubbles, so you need to tilt it up while slowly working the piston in and out....do it slow or you'll create your own bubbles.
That's my .02
Once you have this step verified, pressure bleed each caliper.
Here is how I do this. Cap the MC and use an air compressor like this. I use ~ 20 PSI. But make sure you check the fluid level and dont bleed the MC Dry.
Now the other thing is to check rotor run-out. If this is a high number ~ < 0.008 then it is possible that air is being pumped into the brake lines via the caliper pistons.
A Quik-Clamp fixes the Motive's biggest problem. I've also used this trick with the round universal adaptor on a different car.
You need a runout gauge (Amazon link), a dial indicator on an adjustable arm with a clamp or magnetic base, to test runout. @GTR1999 has posted instructions on this site on how to address the issue with shims.
There are a couple of things the average mechanic is not aware of when working on C3 brakes.
First, like someone else mention, the distance between the back MC piston and the boosters rod is critical. Often the wrong MC is installed because of the wrong year or one MC is for PB and one unit is for manual brakes. They are different in the back, at the mounting casting and the piston depth inside the casting. Perhaps the booster rod was changed also. Who knows?
Second: if a mechanic did indeed bench bleed the MC in a vise and went too far in the casting with a screwdriver, THAT will damage the piston seals. (don't ask how I know) Clearly printed on the MC box is a label: Do Not Push phillips screwdriver Into Bore More Than 1 & 3/8 inch or you will damage unit and void the warranty.
I think that is why your mechanic kept changing out the MC.
Click on my profile and have a look at Bench Bleeding the MC.
I agree with the piston in the MC is critical for proper operation.
I learned about this of course the hard way.
I bought a MC from Napa. damn thing just wouldn't act right. Soft pedal mainly.
I finally compared the new and original MCs and found the piston was completely different.
I basically used the new MC and swapped the new seals over to the original piston.
Great brakes like when car was new.
It has been many many years ago and taught me a very valuable lesson.
The air is interesting. Was the car transported flat or nose/rear high on a carrier?
Maybe some air was in there all along and the cars angle moved the bubble some where to cause the issue.
I had a pesky car do that, drove up a steep drive way and felt the pedal get a little spongy.
Hate these vette brake threads, when in good order they are great brakes, sad when people can't get em to work properly.
I have a 78 and have also done most of these things and once again…pedal to the floor. What is runout and how is it checked?
I really appreciate all the great input. I’ve had a wobble for the 5 years since I bought my ‘78 from the original owner and no one can find the reason, including two Corvette shops?? I haven’t heard anything before about runout and how air could be drawn into the calipers but it certainly makes sense in my case. When I bought the car in 2017 it had been parked since ‘82 with only 27,000 miles but I replaced everything, tires, pads, calipers, rotors, hoses, etc. and had great brakes for awhile but bleeding every year or so became necessary. I’ve got a runout gauge and hopefully today is the beginning of the end to this vicious bleeding cycle. Stay tuned.
11-10-2018, 06:19 PM
1978 No Brake Pressure - Again!
