porschesam

I have a strange problem with the rear brakes on my 89 951. I replaced all four rotors with OEM. New OEM pads, flushed and bleed the systen twice using a pressure bleeder. I also replaced the flexible lines with new. My problem is the rear brakes don't seem to be working. I have a hard pedal and the car stops well enough but the machining marks on the new rear rotors are still visible. After driving for about 30 miles the front rotors are nice and smooth and too hot to touch whereas the rears are only warm and they don't have the smooth surface like the fronts. Any ideas?
This was a problem before I changed the pads and rotors.

ninefiveone

Try the 33 bias valve. The stock 18 bias valve doesn't send much pressure to the rears with the 86-88 baby brembos, never mind your S4 brembos.

Waterguy

Sounds like an ABS problem, especially if it was a problem prior to and after brake service. Have a mechanic perform an ABS function check.

tjbreen

I just did a very similar upgrade. Four new OEM rotors, fout new pad sets (Metal Masters), stainless steel lines, a 5/33 bias valve, and Power bled with ATE blue twice on my '87 to solve a number of problems. Front rotors were below minimum thickness and the backs were wearing strangely (rust visible on the face at the outer edges) due to stuck rear pads. The fronts would lockup at almost any speed if I jammed the pedal quickly (no ABS).

In the rear, the stainless steel sliding plates that keep the pads from pushing through the aluminum caliper were pushed out by something behind them. I ground a little steel off the pad back plate to get free movement of the pad. I tried to get the plates off but they weren't cooperating and the whole caliper would have to be rebuilt for about $250 each but I didn't have the time or cash this season.

I did some tests after break in and I now have what feels like equal heat in both front and rear rotors. Braking has improved tremendously and I have a hard time locking the fronts now. I can't imagine that the S4 brakes could stop this car any better.

I have over 200 miles on the car since the upgrade and all rotors are still showing machine marks. It could be that you have the same stuck rear pad problem and like my car was before, almost all the braking is being done in the front, thus wearing these rotors more rapidly. I have a feeling that my car was driven with stuck rear pads for a while leading to the rapid wearing of the front rotors to below minimum thickness.

Tom Breen