Cheburator

As we increased the power, the pace picked up and then we started suffering brake problems.

1st we rebuild the front GTS calipers - the fluid seals were totally shot, but luckily the pistons were as good as new. The plates are good and there is no lift. There was an immediate improvement in front caliper temperatures - from 260C to 240C. Rear brake calipers - would not even register on the scale - below 150C

2nd - we replaced the bias valve with a 964 55lbs one - front brake temperatures down another 20C, but rears refuse to budge

Using Carbone Lorraine RC6 fronts and EBC Red Stuff rears

Brake fluid - MOTUL RBF660 - bled before every time we go out on track

what happens is that we lose pedal pressure gradually through the race but the car still stops when you stomp really hard on the pedal

I am beginning to think - internal BMC fluid leak...

Any other suggestions?

AO

The "gradually" part makes me pause to confirm your diagnosis.

Are you left-foot "tapping" before your hard braking? You might try and see if that makes a difference. The lateral runout of the rotors will tend to push the pads away and open up the gap. So when you press the peddle, the stroke seems longer/less pressure. Try left-foot tapping the brake pedal a few times just before your braking zone to reset the pads. On the Coke car, I find as the rotors get older, they tend to get worse, an this technique does help - significantly!

If this helps, then it probably is not a BMC issue. I would also try to measure the runout of your rotors to see if they are warped at all. Just be sure the mating surface between the rotor and the hub are clean. I recently added ducting to help cool the rotors. DOn't have enough time yet to say if it helps or not as I just spun a rod bearing.

Cheburator

Mark Anderson

As I grew my front brakes on my race car ( 355mm) I got to the point where I had no rear bias valve.

AO

Having worked in the braking industry in a previous life, there could be a sizing issue of the MC. If you increased the size, the MC may not be able to deliver enough fluid. Now you're increasing the rear bias (I did that too - more on that in a second), so effectively less is able to get to the front. This all spells for a softer. less effective pedal, I think. Of course, your MC could also be bad/tired.

As for increasing the rear bias valve. I replaced the 33 valve on the Coke car and put in a 55. I was warned against doing it, but I wanted to try and get more rear baking, if possible. It really seemed to help stabilize the car on 130mph+ hard braking. And did not seem to negatively affect trail braking (that part i was warned about).

As for the Coke car's health... not looking good. Wrenching tonight. Engine coming out. Going to try and frankenstein a working motor from 2 bad ones. Should have seen the oil - liquid copper.

You must have upgraded the MC to account for the extra fluid in the calipers, no?

Mark Anderson

I did not. I think the Ferrari F50 calipers I used have the same piston size as GTS so no need.

The Forgotten On

If the car was originally a street car for many years before you started racing it I would replace the master.

The cylinder inside of the master can corrode and get some rust on areas that aren't used in normal driving. Especially in wetter climates like yours where water has a higher chance of being absorbed into the brake fluid.

When racing I am sure you press the pedal much harder than any person on the street would need to. This can cause the seals to travel over the rust and corrosion and get damaged.

Possibly causing the issue you have.

Bigfoot928

Per Brembo, the F50 calipers has the bores for the pistons spread out more allowing for better heat dissipation and allows for larger pad in addition to the larger rotor surface. Master cylinder volume needed for both calipers is the same. I have the F50 Calipers on front and the 964 rear calipers and rotors on the rear with a 55 bar regulator. Can't want to see how the brakes feel.