Brake problem? '85 911

[TroyN]

This is for a race-prepped (NASA GTS3 class) car.

At the end of last year my brake pedal was getting progressively lower until at the last event at which I drove, my pedal went pretty much to the floor. After bleeding I get a little more pressure but it only lasts for maybe 1 lap. Sometimes I get more pressure by pumping the pedal but not very often. I have no system leaks, or at least the level in the reservoir does not go down. My pads are wearing evenly.

I've:

- Replaced the master cylinder with a new unit
- Installed new pads (Hawk blues)
- Rebuilt all 4 calipers
- Replaced the soft lines, which were braided, with brand new lines, also braided
- Bled and re-bled numerous times (ATE fluid); combination of Motiv and pedal bleeding

The pedal is still very low. After doing the last 3 items above I haven't had the car out on the track, so maybe it will feel better, but the pedal feels the same or worse than before.

What else is there to look into? These d*** brakes... Thanks.

[gregwils]

Did you bench bleed the mc? You could always remove it (a pain), then bench bleed and reinstall if you didn't initially.

[ivangene]

I have heard that after caliper rebuild the seals retract the piston kind far...the way I recall getting around it is to removed the pads and let the piston extend out and re-install the pad.

obviously you have to be careful not to over extend the piston or it comes out too far... may be a two person thing...

I guess you could check the gap between the rotor and pad first and see how much gap is ni there....and the air idea is valid too.... are you power bleeding? - air can be a problem to get rid of

[chrismorse]

A wild one - are the wheel bearings snug. If there is too much bearing play, the discs are not running true, which causes too much pad knock back, thus using up pedal travel to get the pads to make good contact withthe rotors, every time = low pedal.

Along similar lines, are the calipers mounted square to the disc face.

Another squed possibility - excessive taper wear on the pads.

Wild Ass Guess #2 - is the MC pushrod adjusted too long, not letting the piston return far enough to draw in a full shot of fluid.

And lastly, we must not rule out Demonic Possesion - have you been good lately?????

?? - massively Dazed and confused,
I remain,
Your most devoted servant,
chris

[TroyN]

Thank you for all the replies so far!

Hmm, not sure if I've been good or bad but possession seems as feasible as anything else!

How do you adjust the MC pushrod?

I did bench bleed. I've both power bled and pedal bled. The pads are in contact with the rotors. I noticed a few of the pistons were not exactly 20* which may help with braking effectivness?

I know the brakes on these cars will never be great but I'm just going for decent.

[TroyN]

Oh, I've also spun the wheels and they all rotate nicely, without hitting high spots or warped rotors.

[wwest]

Originally Posted by TroyN

This is for a race-prepped (NASA GTS3 class) car.

At the end of last year my brake pedal was getting progressively lower until at the last event at which I drove, my pedal went pretty much to the floor. After bleeding I get a little more pressure but it only lasts for maybe 1 lap. Sometimes I get more pressure by pumping the pedal but not very often. I have no system leaks, or at least the level in the reservoir does not go down. My pads are wearing evenly.

I've:

- Replaced the master cylinder with a new unit
- Installed new pads (Hawk blues)
- Rebuilt all 4 calipers
- Replaced the soft lines, which were braided, with brand new lines, also braided
- Bled and re-bled numerous times (ATE fluid); combination of Motiv and pedal bleeding

The pedal is still very low. After doing the last 3 items above I haven't had the car out on the track, so maybe it will feel better, but the pedal feels the same or worse than before.

What else is there to look into? These d*** brakes... Thanks.

Since you don't indicate any need to replenish/refill the brake fluid level then I will assume that is not the case.

"..The pedal is still very low.."

If you mean before foot pressure application then the MC return spring is a problem, or there is some frictional effect that prevents a full return of the MC/pedal. If you mean the gas pedal travels closer to the floor AFTER foot pressure application then:

Your MC is allowing "back-flow" as you depress the brake pedal. If the brakes get progressively worse over a day of track use then try somehow cooling the MC.

[CCM911]

The last time I rebuilt my calipers, I stupidly let the MC run dry. It took me several bleed sessions to get the pedal to feel right. It would be great if you could run the car around the block a few times to exercise the brakes, and then re-bleed.

[TroyN]

How does "back-flow" occur in the m/c, and how do you fix it?

I should drive it around some and re-bleed.

Thanks.