OriginalSterm

At a DE a few weeks ago I experienced a loss of brake pedal going into the bus stop at WGI. I bled the brakes pretty well after that, and the pedal came back. A few laps later, and the pedal was soft again. I bled the brakes yet again, but the pedal stayed soft the rest of the day. Is it possible the brake fluid I was using is no good anymore? I opened it at the end of last season and kept it sealed in a ziplock bag, but it has been open for quite a while now. Is it beyond it's usefulness? Should I buy new and bleed the heck out of everything?

Brake pads are at ~75% front, ~90% rear (PFC97)
Rotors are a bit older, but I don't think they are at the limit (which is how thin?)
Extra ducting to the rotors to help cooling
Ate Super Blue

cooz

based upon your description and equipment...


i would start with new brake fluid...i now use castrol srf....

then possibly a new 100% front pad.....

look for any possible reasons that may be causing excessive heat build up like dragging etc..make sure brake ducts are in place, etc...may want to check temps with electronic pyrometer...

do you have abs?

kurt M

I would think that there was still air in the system. A fluid will not compress enough to mater in a brake system but a gas will and will make a soft pedal. Even old fluid does not compress.

If this were my car before I switched to SRF I would rebuild all 4 corners and the MC if they have not been in the last year and sooner for track driven cars. Shame to waste the fluid after you cook an old seal.

Professor Helmüt Tester

Check the fronts for pad taper, and check your wheel bearings.

OriginalSterm

I didn't pull the pads yet, I will check them for taper. The wheel bearings are pretty new and seem fine, but I will look at them again. How would they be related to a soft pedal? Is that what would cause pad taper?

TR6

If the wheel bearings or spindles are allowing the rotor to 'wobble' due to excessive play, the wobble will cause the pads to not contact the rotor uniformly and the pistions on one side will actually push out further. This translates to what feels like the brake pedal going down farther to the floor than it should to stop the car. I've had it happen to me. Ironically, it was immediately following a wheel bearing replacement (quality of work issue). I swore it was my brake fluid going bad until I jacked up the car and wobbled the front wheel. Yikes.

Professor Helmüt Tester

As Greg pointed out above - a bad bearing will cause similar braking problems to a loose wheel - rotor will wobble on the Z axis. Rotor wobble causes 'pad knockback', and you've got to push the pads back out into contact with the rotor each time you reapply the brakes.

Jack up car, grab top and bottom of the tire and rock it in and out...if you feel any play, then there IS play.

Taper can be due to poor cooling, bad heat range of the compound, or the aurora borealis. More likely to cause soft pedal on brake calipers with >2 pistons, but it can be 'feel-able' on any caliper.

SundayDriver

If you are only feeling it in one spot on the track, then it is likely pad knockback. Maybe you are hitting curbs in one spot or the g loads are such through the esses that you get more rotor wobble.

chrisp

To expand on Mark's point...

Considering that it's happening going into the bus stop, depending on how hard you brake for turn 1, your pads could have been cooling all the way from turn 11. With a well ducted race pad that would definitely give a longer than normal pedal.

OriginalSterm

Let me be a little more clear than my initial post. The bus stop is the first place I lost brakes, each run following that issue I experienced a soft pedal in most (if not all) braking zones. Bleeding did not help solve the problem. Another 951 driver noted how soft the pedal felt when stepping on the brakes while parked in the paddock.

Excellent explanation of pad knock back and the interaction between wheel bearings (bad) and braking. Thanks!

chrisp

thanks for clarifying. sounds like air. did you bleed the clutch as well?

OriginalSterm

No, I did not bleed the clutch at the track (or this season come to think of it). Would that effect the brakes, I assumed I would only feel it in the clutch pedal?

Professor Helmüt Tester

You may have a funky master, or something going on in the rear brakes.

Bleed master. Check proportioning valve. Bleed rears. Bleed rears again (those lines are long, and if you've got air in them, it has to go a long way to the bleeder).

Excessive brake cooling won't give you a long brake pedal...it'll just give you 'wooden' brakes - a hard pedal that doesn't give much decelleration.

aeshultz

One side note on wheel bearing play in 944s - in the early cars (not the series II) I've been told that some play is normal, at least in the rears. Happened to me at the Mid-Oh club race - I had rear wheel virbration in a practice session, put the car in the air, everybody said, "yep, bad bearing." Took the car directly to Steinel's, put it on the lift, Eric and his best Tech both looked at it - "Nope, this is normal - VW bearings." Back to the track, lession learned.

Professor Helmüt Tester

Typically, there is no adjustment on the bearings on the 'driven' wheels. "Assembled bearing", pressed into a knuckle, then a stub axle pressed into that.

The OTHER end of the car...the 'non-driven' end, generally has bearings that allow for 'pre-load' adjustment. Description of this adjustment should be in your shop manual.