Curt wrote to the list:

Hi, all. So I've got this strange situation with the brake pedal feel on my '86.5 5-speed, and I'm hoping this sounds familiar to someone  out there. The feel of the brake pedal is very firm, except for the occasional time when it's not. :(

During that occasional occurrence, the pedal is slack for about an inch (2.5 cm) before it becomes firm.
Instead of being firm immediately on application as it usually is, there's little or no resistance to the pedal for that first inch,  maybe more, before the brakes start to bite. Lifting off the brake pedal and re-applying immediately brings it right back to its normal,  immediately firm feel. This is not an on-the-track scenario, this is merely maneuvering in the parking garage.

I've just determined how to reproduce the symptoms, and can do so without fail: just make a slow, tight turn.
It doesn't matter whether it's to the left or to the right, the slow, tight turn ensures that the next brake application will have this initially slack condition. The symptoms do not occur when driving in a straight line.
I cannot associate the appearance of the symptoms with any particular event.

More details: a couple days ago I had new brake pads, discs, and SS lines installed at each corner and the brake lines thoroughly flushed.
The symptoms occurred both before and after, there was no change in the symptoms with the installation of new parts. Normal usage for this car is track and autocross, and the brakes generally work great.
There are 129k miles on the car, but records show the calipers were rebuilt somewhere in the 105k-115k mile range (I forget at the moment).
 

Any ideas? I have none, but hope to be able to reproduce the condition with my mechanic later this week. Thanks, Curt
=========
First of all, great post and diagnostic method so far.

The "typical" cause of pump-up pedal syndrome is a master cylinder problem with the bypass check valve. But the "happens only after turning" symptom points to something at the wheels instead. Sounds like the pistons are retracted a little extra by something when you turn, and this says "wheel bearing adjustment" to me. Reasoning: With the calipers mounted towards the front of the steering, looseness in the wheel bearing translates into rotor movement when you turn the steering wheel. That rotor movement pushed the pads back slightly into the caliper, also pushing the pistons back, forcing a little bit of fluid back up to the master cylinder. With the pads slightly retracted, it takes a little extra pedal movement to take up that clearance before the pads are again in firm contact with the rotors.

If the wheel bearings are fine, then the next look is at the master cylinder itself.

******

The SoCal board has had a recent discussion on bleeding/flushing, and I was reminded that the flushing operation includes opening the bleed port on the front of the master cylinder as the first step. With that valve open and the reservoir slightly pressurized by the power brake bleeder, you can stroke the pedal --gently-- until almost full stroke to maybe free up any loose bits of dirt/rust/scale that might have built up in the master cylinder itself. If you bottom the piston hard in the bore there's a risk of rolling a cup, but a gentle push and an almost-to-the-bottom stroke length should be fine. After a few of those, increase the bleeder pressure for a good purge. You can then replace/close the master cylinder bleed valve and proceed to flush the lines to the wheels as normal. At the calipers, unbolt them and flip them upside down for the first flush seconds. Put a piece of plywood or something in between the pads to keep them from extending. Any crud should then fall to the bleed holes and get pushed out.
Remount the caliper correctly, restore the bleed valves, and then bleed any air out, and you should be reasonably sure that the system doesn't have any big rocks in it.

I know this is your race car and it gets very regular fluid changes/flushes so this may not apply directly to you. It's the next logical step before rebuilding the master cylinder though.

HTH!

dr bob
'89 S4 in SoCal Winter Hibernation
 

=========

May be loose wheel bearings. If the bearings are very loose, the brake rotor can cock enough to push the pads back into the calipers. Upon the next application, to properly functioning brake system has to first push the pads back into position before they hit the rotors and give resistance.

Wally Plumley
928 Specialists