Brakes fail when engine is hot

[Alfonso Gomez]

On february I posted my problem with the brakes. In a nutshell, when the engine hit 205-210 degrees during a nasty traffic jam, the brake pedal went down to the floor. After sitting for an hour, the brakes were back to normal. The general consensus was either heat soak or contaminated brake fluid. It did happen once more about a month after thet first time (in March) and after cooling off for half an hour the brakes went back to normal.
I started avoiding using the car too much until in April I took it to a GM dealer and had the brake system purged and the brake fluid changed (Wynn's Friction Proofing System). They used a pump to take out all the old fluid and put in new fluid.
This weekend I took my wife for a ride (90 degree weather), got in a traffic jam, the coolant temperature went up to 215 degrees and the brakes started to fail once more. I checked the fluid and it was clean and clear, not muddy or dirty. It was very hot and the master cylinder was VERY HOT. My idea is the the master cylinder gets too hot, expands and the rubber "piston" inside is not sealing properly.
Is this possible?

 

[grapeknutz]

First I would change out the brake fluid, DOT3 has a bad habit of absorbing water and it could be gassing in the master cylinder or brake lines.
Brake fluid should be changed out once a year, just to keep the piston calipers from rusting out. Again because of mosture!

 

[Alvaro]

Alfonso:
I have the exact same problem, and I have the same idea You are mentioning. I already got the new master cylinder but I really want to see the comments from the experts.

Cheers from Mexico.

 

[Ken]

Damn! Maybe it's a "Latin Thing". :L

Sorry man, I couldn't help myself.

 

[vettepilot]

I wonder if the master cylinder piston seals are beginning to deteriorate, and under increased heat and pressure the seals fail to seal, allowing the piston to go full forward.
I don't see how the fluid could get hot enough to cause this, that type fluid has been around for many years withstanding underhood temps. Unless you have a brakeline that somehow was displaced so it is now very close to, or on top of, an exhaust component. Other than that, I don't see how there could be enough heat transfer to cause it to gas. That usually comes from repeated hard braking on the track, but certainly not in stop and go traffic situations.

vettepilot

 

[Hrtbeat1]

booster heating up

One thing to check would be the vaccuum on your brake booster. Once it heats up you maybe loosing vaccuum. Just general physics heated air is less dense than cold air, ie less vaccuum. I saw an article once about Guldstrands '86 (?) that he was doing testing at Talledega with a 383 and he used heat barrier material around the engine side of the booster. Yes it looks tacky but I'm sure it looks better than a Vette that couldn't stop.

Hope it helps

 

[c4cruiser]

I was under the impression that a loss of vacuum in the booster would cause a hard pedal, not a soft pedal much less having the pedal going to the floor. Underhood temps should not really reach more than 150-200 degrees so the brake fluid should be nowhere close to boiling. Even the cheap brands of auto parts stores DOT3 brake fluids will handle 400 degrees.

I would suspect the master cylinder piston seals. Try replacing the master cylinder to see if that solves the problem.

 

[Hrtbeat1]

Failed booster

The only reason I guessed the booster is from a previous problem I had. My booster ruptured and I had almost no brakes. Pedal went to the floor and under very hard brake pressure (two legged press) I was able to stop. The brake booster is a plastic part and very susceptible to high temps (not to mention pressure.) The enclosed pic shows my failed booseter inverted, from a visual inspection you couldn't see the crack as it was toward the engine on the lower side.

If the only times you have seen this problem is when the temp goes up I would guess your's is not as bad as mine was. High pedal may indicate a failed seal in the master cylinder (the vaccuum has no where to go), but low pedal would indicate something either leaking from the booster or lack of vaccuum in the booster (my problem.)

Your car getting up to that temp at idle (restricted air flow due to traffic) would lead me to believe the radiator chamber may be dirty. Check some of the posts on cleaning the radiator you would be really supprised at the birds nest that you may find in there. Other than that I would guess the booster is getting hot and reducing the vaccuum pressure (you did say the master was VERY HOT.)

C ya

 

[vettered91]

Damn! Maybe it's a "Latin Thing". :L

Sorry man, I couldn't help myself.

You better believe it Ken!

I had the same problems before with the '91 and now with the LT1 and I'm almost sure Alfonso is right about de "O" rings inside de master cylinder, but I'm waiting until Alvaro puts on the new one and see if it solves the problem. (Please keep us posted Alvaro).

Alfonso, you just did everyting but the Master Cylinder so it must be the problem, right now I can't do much in my vette because it's been 15 days at the shop because I blew up the torque converter and the transmission oil pump!!! :cry

Good luck

 

[Alfonso Gomez]

You better believe it Ken!

I had the same problems before with the '91 and now with the LT1 and I'm almost sure Alfonso is right about de "O" rings inside de master cylinder, but I'm waiting until Alvaro puts on the new one and see if it solves the problem. (Please keep us posted Alvaro).

Alfonso, you just did everyting but the Master Cylinder so it must be the problem, right now I can't do much in my vette because it's been 15 days at the shop because I blew up the torque converter and the transmission oil pump!!! :cry

Good luck

A Latin Thing !!!!!!! The trouble here in PR is a lack of really good Corvette mechanics, so I rather do it myself, even if I break something. Only things I don't tackle are engine internals, trannies and certain suspension items that require a lift and/or special tools that I don't have.
Back to the brakes, I found that the front brake hose on the passenger's side was twisted and had blisters on the outside (I did not install that one). So I got a new one, changed it and bled the system. Now I have to find a really nasty traffic jam to try the brakes

 

[Alvaro]

I will make the change of the master cylinder in a couple of weeks, a friend of mine has to make some space in the place he works (Audi dealer) to work in my car.
Alfonso: Just like in PR, here in Mexico we don't have really good mechanics for Corvettes, I also make the small repairs my self, hope not have to work with the internals of the engine very soon.
I'll let you know the results of the new master cylinder.
Any suggestions about cautions when bleeding the lines-ABS unit after the installation of the new cylinder?