Jimbo951

I have an 86 951 and I’m struggling with a soft brake pedal. It feels like classic pad push back. I can have a firm pedal, and then go through a corner and it’s soft at the next braking zone. To compensate, I tap the pedal on the straight and build pressure for the next corner and this works, but it’s really, really impacting my driving.

This is not a brake fade/cooling issue. I’m running SRF fluid and I have a soft pedal during the first lap.

In the past year we’ve replaced:

Rear wheel bearings
Master cylinder
Rear Trailing Arms and more new rear wheel bearings and stub axle (found aluminum shavings in the trailing arms and determined the bearings were wobbling around inside the trailing arms)
Transaxle mount (found root cause of previous problems: Transaxle mount had failed and the transaxle was moving side to side, pushing the half shafts into the stub axles/wheel bearings. Also found damage in the diff! Replaced OEM mount with a solid mount)

It's possible when the rear bearings were moving around that the brake rotor contacted the caliper body.

Still had problems this weekend. Found 1 stub axle nut had loosened about 30 degrees. We re-torqued both rear axle nuts. Brake pedal was slightly better but not great.

Things we have not change: Calipers (I think these are original) and brake hoses (they’re SS Braided hoses, about 12 years old).

When I test the wheel bearings by pushing/pulling the top/bottom of the tires, everything feels firm.

I’m struggling on how to identify and solve this problem. Seems like we’re just going to start throwing money at new parts until we fix the problem.

Any suggestions on what to do next?

Thanks.

ltusler

I'd rebuild the calipers and change those hoses.

Deadeye

I had similar problems for 2 seasons. Last winter I replaced the front wheel bearings, the rear rotors and rebuilt the rear calipers. Not sure which one was the cure, but no more knock back.

fhp911

slight digression but this seems a useful place for my question.

WHY is my pedal soft at the beginning of each session? (I'm not sure about the 1st session of the day.) Then after Turn 1, it is hard. ( I've had this effect on several of my track cars over the years.)

Residual heat from the last session maybe? If so, why would that matter?

Other ideas?

Matt Romanowski

I've seen a couple of Boxsters have the vacuum booster go bad. Could be in there.

TrackDays247.com

Rebuild the calipers - not toooo hard but parts are $$

teamking

Jimbo,

One option for rebuilding the calipers is to get late brembo's off of an S2. They are essentially the same calipers (one of the piston sizes is slightly different), but the late brembo's are much less expensive to rebuild. So, buy them, rebuild them, take the old calipers off, and sell them.

As to other ideas: have you checked the runout on your rotors? (Didn't see that written up)

Jimbo951

Rotors are new, only 6 track days on them.

Van

Unless you're the Hulk, you won't be able to exert as much force on the wheel as a 3,000 lb car cornering at 1.25 Gs.

Lol - for some of us, 6 track days would be 1/2 used up!

Jimbo951

6 days is about 1/3 used up.

And I hear ya on the Hulk comment. That's one of the numerous reasons this problem is so difficult to find.