11 replies to this topic

[LarryKing]

At VIR Saturday I was racing in the 65 minute enduro. At about 20 minutes into the race I pitted for the mandatory 5-minute stop. Brakes were normal when slowing to the 40 mph pit limit and stopping at my spot. I rejoined the race after 5 minutes and when I got to turn 1 the pedal went to the floor. After backing into the wall I drove to the gap in the guardrail. Still no brake pedal. After sitting 20 minutes I checked the pedal again and it was normal (very firm). Pads are 75% good on all corners. No leaks seen on visual inspection. I have not opened the bleeders yet but I suspect I will find air bubbles.

 

I spoke at length with Bennett (a brake engineer) and he ruled-out boiled brake fluid, which was my first theory. He thinks that air entered the system somewhere, likely the "accordion" boot and/or piston seal. We are both perplexed by the self-healing of pedal pressure.

 

Any theories welcomed.

 

[Alberto]

That's definitely an oh shit moment... 

Following for answers

[Bench Racer]

Seems heat related. During your 5 minute mandatory stop did you continue with foot on brake pedal, heat transfer, boiling brake fluid?

[Steve Scheifler]

Man, that’s one elaborate story to get off the hook with the rest of the team!

This is a looong shot because I can’t really make it fit but there is another device in the system to consider, that pesky little brake bias valve on the firewall.

[Jeff Wasilko]

I'm pretty sure I had a bias/proportioning valve fail and send me into the wall at Watkins Glen T1. I have the Iron Canyon brake pressure sensor adapter in my NA, and it logged that I had brake pressure, but I didn't slow down at all.

I'd replace the master and prop/bias valve for sure.

[Steve Scheifler]

I'm pretty sure I had a bias/proportioning valve fail and send me into the wall at Watkins Glen T1. I have the Iron Canyon brake pressure sensor adapter in my NA, and it logged that I had brake pressure, but I didn't slow down at all.

I'd replace the master and prop/bias valve for sure.

Interesting, but was the pedal soft or hard? I’m having trouble with it causing the pedal to go to the floor.

[LarryKing]

I never hold the brakes when they are hot - doing so transfers pad material to the rotors that creates a huge vibration under braking.

 

Brake bias valve is original to car, so 32 years old - may be worth replacing.

 

I'll know more once I get time to work on car.

[Jeff Wasilko]

Interesting, but was the pedal soft or hard? I’m having trouble with it causing the pedal to go to the floor.

In my case I think the pedal was hard....it's been a few years.

[Steve Scheifler]

I never hold the brakes when they are hot - doing so transfers pad material to the rotors that creates a huge vibration under braking.

Brake bias valve is original to car, so 32 years old - may be worth replacing.

I'll know more once I get time to work on car.

On one of our 1.6 cars we went through a “keep replacing shit until it gets better” sequence because it just didn’t have the pedal feel and stability of its stable mate. What finally fixed it was replacing the bias valve. Although difficult to express exactly what changed the effect was immediately obvious under threshold braking, it just seemed to hunker down and shed speed without drama. It was then decidedly better than the other car so I immediately ordered one for that but it was never quite as good. Braking really is a sum of the parts and balance thing.

[Tom Sager]

Larry I had a similar experience at Road America a couple years ago.  The pedal went to the floor entering a corner and then was good the rest of the session.  We bled them after and all was fine next session.  Sunday afternoon while driving to the grid the pedal again went to floor.  I then found a slight leak at the master cylinder.  Replaced that and life has been good brake-wise since.  Strange for sure but as others have said, many of our cars are running an old brake master, proportioning valve, brake booster vacuum hose (which is valved), brake booster and lines.  If you have a 30 year old master, I'd replace that even if you find another problem.     

[LarryKing]

Yep, my car has the original master cylinder - I plan to replace it.

[LarryKing]

Post mortem: I believe lazy maintenance is to blame. My theory: A combination of an inadequate gravity bleed and failure to flush the several season old brake fluid both contributed to the failure. I replaced all bleeder screws, which all were corroded and then pressure bled the system with new fluid. Left front fluid was very dark (burnt?) and left rear bleeder screw hole was partially occluded by corrosion. I subject myself to public shaming so that others may learn from my stupidity. Lesson learned - mandatory full flush before each season.