81 Brake Mystery
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5th Gear
Joined: Sep 2004
Posts: 5
Likes: 0
81 Brake Mystery
New master cylinder installed in January. Lines were bled on 4 different occassions due to loss of pedal(I understand this is common on this car). No fluid loss ever detected. All fine for 2 months, then after 150 mile trip pedal gone. Stopped for service - no fluid loss, brakes bled 3 times to get a pedal. Back on the road pedal gone after 10 miles. My mechanic says he does not know of any reason for air in the system. I need advice from the experienced 81 owner or mechanic.
Le Mans Master
Joined: Jan 2000
Posts: 8,918
Likes: 27
From: New Hill NC
This is a very common problem and there should be tons of information in the archives both here and the NCRS site to read. The fact is, you have runnout in one or more rotors (most likely the rears), and the calipers are pumping air. You will need to check and take care of the runnout to solve the problem. OH, and get a new mechanic that knows Corvettes.
Le Mans Master
Joined: Aug 2000
Posts: 9,341
Likes: 0
From: Las Vegas Nevada
I had exactly the same problem after i put new pads and rotors on my 81'. As it turns out I eventually discovered that I had to much runout in the new rotors which caused the rotors to "wobble" in the calipers which pushed the pads around unevenly and let air into the break lines.
If you've replaced the rotors recently it is almost required to find a shop that can machine the new rotors on the car. The mechanic that finally told me what my problem was said that there could be no more then .010in (ten thousanths of one inch) of runout in the rotors or it would cause exactly the problem you described and I had. He set the car up on the lift, removed the wheels and machined all four rotors to get the runout out of them and my problem was solved.
You didn't say if you've done a break job on it recently but I'd bet you a beer thats what your problem is if you have.
If you've replaced the rotors recently it is almost required to find a shop that can machine the new rotors on the car. The mechanic that finally told me what my problem was said that there could be no more then .010in (ten thousanths of one inch) of runout in the rotors or it would cause exactly the problem you described and I had. He set the car up on the lift, removed the wheels and machined all four rotors to get the runout out of them and my problem was solved.
You didn't say if you've done a break job on it recently but I'd bet you a beer thats what your problem is if you have.
Safety Car
Joined: Feb 2002
Posts: 4,286
Likes: 4
From: Guam
Actually, I think that you might have a bad check valve in the master cylinder. Some master cylinders have sat on the shelf too long and as a result have some corrosion in the bores. Perhaps, you have one that is defective. Ask your mechanic to check the master cylinder check valves and seals by dismantling it and visually inspecting it.
Drifting
Joined: Sep 2004
Posts: 1,270
Likes: 4
From: Celeste Texas
Go for a drive and use the brakes. Stop somewhere and quickly check your brake fluid. If it's foaming then you have a run out problem. Also have your brakes bled from both ports on each caliper. Quite a few corvette calipers have two bleed ports so you can get each side bled. If you only bleed one then there is still air inside the caliper.
Bill
Bill
Last edited by texaswilkins; May 31, 2005 at 07:36 AM.
