Triton170

Need some help with my 78’s brake issue. I have rebuilt the brake system but can’t get better than at best 1/2 pedal. Here is what I’ve done:
•New master cylinder (bench bled and booster plunger adjusted to 1/16 inch clearance)
•New brake lines (all hard lines and 4 braided stainless steel flex lines)
•All 4 calipers rebuilt and pressure tested
•New rotors and pads
•Pressure tested vacuum booster
•New “proportioning” valve – kept switch centered when bleeding brakes
•Bled system 4 times in proper sequence (manual, gravity, speed bleeders, and Motive Power Bleeder)
•Ran over 2 quarts of fluid through the system and all discharged fluid is clear with no air bubbles.

Entire system is dry with no leaks in lines master cylinder or calipers.

What else can I do? Pedal goes almost all the way to the floor. I am totally baffled.

Peterbuilt

That is too much space, you want just a few thousands gap.

Triton170

Thanks. I'll give that a try.

Triton170

I adjusted the booster plunger to about .005 inch. Only helped the pedal by about 1/2 inch. Still baffled.

HeadsU.P.

Peterbuilt is right. You are at 0.062 at the end of the booster rod and I think you are shooting for 0.015. But I suspect that's a minor issue. Nine times out of ten, there is still air bubbles in the MC and maybe one or two bubbles in each caliper.

When you bench bled did you tip the MC or keep it flat in the vise?
Check out some photos in my album on bench bleeding.
You didn't push the screwdriver in too far or too hard on the MC in the vise did you? That will damage the piston seals, void warranty.
1 3/8" max with the pump stroke in the vise.

Street Rat

I'm having the exact same issue on my '81.

Been there and done that. Still a spongy pedal.
I've really had it with this crap. I did tilt my mc up , down, and level many times. I'm thinking I got a defective mc when I bought it. I'm going to put some plugs in the mc ports and isolate the issue.

theandies

Start bleeding at the right rear, then left rear, right front, left front in that order. May not solve your problem but over the years and manually bleeding brakes on many cars I've found this is the best method.

Also did you totally drain the old fluid out of all the body lines and if so how long were they exposed to atmosphere? If you are in a high humidity area etc moisture may have entered the lines and condensed in the bends. Many bends can produce a significant amount of water. You might consider flushing out the lines with something other than brake fluid.

I'm throwing out this obscure stuff to consider as it appears you've done everything correctly and through. Sometimes its the obscure stuff that drives you mad.

Peterbuilt

Rear calipers have two bleeders each.
Did you bleed the inners first then the outers?

Tapping with a small hammer on the calipers help loosen up any stubborn air bubbles.

cagotzmann

You may want to try and bleed the MC while on the car.

Here is what I discovered during the years of using my car for track days where I bleed / check the brakes after every track day.

Here is what I posted.

https://www.corvetteforum.com/forums...-cylinder.html

Sky65

Did you replace all those parts to repair the condition you have or is it new after replacing the parts? You have about 1/2 pedal. Does it pump up and improve then go away or not pump up and is just low? I don't know about c3s but C2s have two different pedal push-rod locations. One for manual and one for power. Are you in the right one?

Tom

Rescue Rogers

check the numbers on the MC and make sure they sent you a power brake master and not a manual.

Fredtoo

A few thoughts....

What kind of brake fluid are you running? Silicone fluid tends to keep air bubbles in suspension.

Have you driven the car on the road yet? How many miles?

By driving it, you will do two things.

These rigid mount calipers work best when everything is square and true to the world. Bedding in the brakes will true things up, and wear the surface of the pad so it sits closer and parallel to the disc.
and
The heat and vibration helps to break free any air bubbles that are stuck in the caliper.
A quick bleed job after driving the car will usually get the last of the stubborn air out of the lines.

Triton170

Thanks for all the info/help.
• All the lines both hard and flex are new.
• The condition didn’t exist before I replaced the entire system as part of a vehicle restoration.
• Master cylinder was tilted several different ways when bench bled.
• Tapped the M/C and rotors with a brass hammer when bleeding to shock any bubbles loose.
• Road tested the car for a total of 11 miles.
• M/C is the correct power brake version with a 1.125” (1 1/8) bore.
• Bled rear inners before outers
• Valvoline Synthetic DOT3/4 brake fluid
• Pedal does not pump up. It stays at 1/2.
• Will try CAGOTZMANN’s method of bleeding the M/C while it is on the car.

SwampeastMike

I went through similar after replacing the soft brake lines at the calipers. After bleeding and bleeding and bleeding via every way available to me (vacuum pump, two-person, gravity). With very energetic pumping during the two-person method we could occasionally get a full, firm pedal but it went away very rapidly even when not changing the bleeding point.

On the second day and out of desperation I assumed that the master cylinder somehow had some air trapped in it. I covered fenders and everything in the engine compartment, left the master cylinder cap off and had my helper slooooowly press the brake pedal to release when I shouted. Sure enough I saw tiny bubbles come up through the holes in the bottom of the master cylinder just before the stream "jumped" out. We did this a few times, bled everything with the two-person method and repeated at the master cylinder. I then waited overnight, bled using the hand vacuum pump and getting a decent pedal took it for a test drive. After a very few stops I had a perfect firm pedal and it has stayed that way since.

Sky65

I found this. You have already done allot of it but maybe something will jump out at you.

http://www.classicperform.com/TechBo...ot.htm#pushrod

Tom

calwldlife

it is not uncommon for the parts house to give a wrong m/c
rebuilder of m/c could mark it wrong.
or a generic part was selected by seller.

i am not talking bore size but the first piston of the m/c that the brake pedal rod hits.
there are subtle things about them.
hope you have the m/c that worked before the change.

cagotzmann

Did you have the MC level setup so the back of the MC is higher than the front. If you had you would have gotten all the air out. Lifting the back of the car so the MC is higher at the back than the front makes the air settle against the MC rod seal just behind the hole in the bottom of the MC. Then when you press the brake slowly all the air gets removed. If you didn't lift the back of the car not all the air will be removed. See my above post with a link to a video I found demonstrating how to bleed the MC while still on the car.