Help troubleshooting my brakes on a 69 coupe

[Michael L.]

Guys,

I'm finishing up the frame-off on my 69 coupe and I'm having trouble getting any pressure in the brake pedal. The calipers were rebuilt a long time ago (?15yrs) by lonestar with stainless steel sleeves and the master is a recent (2-3 yrs) purchase of a date coded piece from one of the vendors. I researched the forum extensively but ultimately decided to go with DOT 5 fluid mainly because I didn't want to risk any damage to the paint with the inevitable spill. I extensively bench bled it before the install and I used the motive products pressured brake bleeding tool to bleed the lines. I see no evidence of leakage anywhere. No idea what the problem could be. Any suggestions?

Mike

[Richard K.]

Re: Help troubleshooting my brakes on a 69 coupe

Myself, I would suspect the master cylinder having an incorrect re-boring, and also the size of the seals used. A start in tracing the problem hopefully. Good luck.

[Joe L.]

Re: Help troubleshooting my brakes on a 69 coupe

Guys,

I'm finishing up the frame-off on my 69 coupe and I'm having trouble getting any pressure in the brake pedal. The calipers were rebuilt a long time ago (?15yrs) by lonestar with stainless steel sleeves and the master is a recent (2-3 yrs) purchase of a date coded piece from one of the vendors. I researched the forum extensively but ultimately decided to go with DOT 5 fluid mainly because I didn't want to risk any damage to the paint with the inevitable spill. I extensively bench bled it before the install and I used the motive products pressured brake bleeding tool to bleed the lines. I see no evidence of leakage anywhere. No idea what the problem could be. Any suggestions?

Mike

Mike-----


I'll repeat something I've said many times before: there is absolutely no service operation on a C2-C3 Corvette that I fear and dread more than bleeding the brakes. NONE!

[Richard M.]

Re: Help troubleshooting my brakes on a 69 coupe

Mike, Was it good pressure when you first bled the system? Did you inspect all calipers for leaks?

If so, it sounds like the master is bypassing, i.e. a internal leak. Best to remove it and open it up to inspect seals, etc.

Rich

[Bob I.]

Re: Help troubleshooting my brakes on a 69 coupe

Gravity bleed a wheel to verify that no air is present. I also used Dot 5, in my 1970, I experienced a rear seal leak in the MC as it leaked out the port in the master cylinder body. Bench bleeding the MC is a timely process, you need to take time in filling the MC with Dot 5 so no air is introduced.

Bob

[Michael L.]

[David H.]

Re: Help troubleshooting my brakes on a 69 coupe

... Bench bleeding the MC is a timely process, you need to take time in filling the MC with Dot 5 so no air is introduced..

Michael

Did you BENCH BLEED your master cylinder?

Dave

[Leif A.]

Re: Help troubleshooting my brakes on a 69 coupe

Michael

Did you BENCH BLEED your master cylinder?

Dave

Dave,
Quote from the OP original post:

"I extensively bench bled it before the install"

[David H.]

Re: Help troubleshooting my brakes on a 69 coupe

Dave,
Quote from the OP original post:

"I extensively bench bled it before the install"

Leif

Missed it on first read, thanks

Dave

[Michael L.]

Re: Help troubleshooting my brakes on a 69 coupe

So I am trying to gravity bleed the brakes. I've opened all four bleed valves and...nothing is happening. Not a drop of fluid has come out yet. I also removed the MC cover as I figured it would create a vacuum that would impede the bleed. It's been about 20 minutes already. How long does it take?

Mike

[Tom R.]

Re: Help troubleshooting my brakes on a 69 coupe

I replaced M/C and calipers a couple of years back and tho I bench pled the M/C still had a problem until I stumbled upon the brake pressure-differential valve that needed to be reset. Easy in 78, given the relief valve could be reset from an external port. However I know this did not come about until 78...not sure what the early C3s had but you might look to it...I don't have the 69 manual but suspect they were in use to balance front to rear brake pressure.

[John P.]

Re: Help troubleshooting my brakes on a 69 coupe

Hello Michael, I noticed that your post stated you opened all (4) bleeders to bleed the brakes. The rear calipers each have (2) bleeders making a total of (6) bleeders that need to be opened. If you were trying to bleed the system without bleeding all (4) rear bleeders, you will never get a good pedal. If that was the case that you only opened (2) rear bleeders, attempt to manual bleed the system with an assistant. Have the helper press the pedal down slowly and allow to return slowly. Do not pump the pedal with DOT 5 fluid as it tends to aerate the fluid. John

[Joe L.]

Re: Help troubleshooting my brakes on a 69 coupe

I replaced M/C and calipers a couple of years back and tho I bench pled the M/C still had a problem until I stumbled upon the brake pressure-differential valve that needed to be reset. Easy in 78, given the relief valve could be reset from an external port. However I know this did not come about until 78...not sure what the early C3s had but you might look to it...I don't have the 69 manual but suspect they were in use to balance front to rear brake pressure.

Tom------

The feature you describe is unique to 1978-82 Corvettes. Those model years were equipped with a combination valve. This valve combines brake fluid distribution, brake system low pressure warning, and fixed rate system proportioning. 1967-77 Corvettes used only a brake fluid distribution and low pressure warning block. On these models there was no differential "balancing" between front and rear systems.

By the way, 1965-67 Corvettes with J-56 brakes were equipped with an adjustable proportioning valve.

[Bob I.]

[Michael L.]