Brake Fade

Jeeper31 · 02-25-2007, 10:26 AM

On my '88 951S after a sharp turn my brake pedal seems to fade when I step on the pedal. The next press on the pedal is fine and it is always fine when driving in a straight line. It just happens after a turn. I checked the front wheel bearings and there was some play in the wheels so I changed both front inner and outer bearings. The wheels are tight but the problem is still happening. Any ideas?

TIA

toolboy62 · 02-25-2007, 10:32 AM

How about the rears? It still sounds like the rotors are moving WRT the calipers and knocking the pistons back.

nate

Jeeper31 · 02-25-2007, 10:35 AM

Quote:

Originally posted by toolboy62
How about the rears? It still sounds like the rotors are moving WRT the calipers and knocking the pistons back.

nate

Wow, stupidly enough I didn't even consider the rears. I just assumed since it was in a turn it would be the fronts.

Jeeper31 · 03-11-2007, 06:03 PM

I checked the rears and they are good. I readjusted the front bearings and they are tight now. The problem is still happening. To narrow down where it's coming from I let the car stand still idleing. I then turned the wheel all the way to the left, then all the way to the right then back to senter. I pressed on the brakes and the pedal was low again. Pressed it again and the height was fine. Turned the wheels again without moving and it was low again. Next press it was fine.

Any more ideas?

HondaDustR · 03-11-2007, 09:31 PM

Maybe the rubber brake line fittings are loose and they sort of loosen/tighten while the wheels turn? I don't know, It just came to mind...

Jeeper31 · 03-12-2007, 04:01 AM

Luckily, the previous owner installed steel brake lines.

SoCal Driver · SoCal Driver's Garage

Front wheel bearings should never be "tight". Should be a slight bit of play in them. Check that the whole caliper housing is free to move in and out. I've had the rear bind on mine and wear the pads into a wedge shape.

rudidelange · 03-12-2007, 07:34 AM

Turning a wheels should not theoretically affect the brake pedal. I experience the same effect when pressing the pedal the first time, less when pressing the second and third time. The braking ability was less than normal and the brakes felt "spongy". It turned out to be one seal inside the brake master cylinder that was leaking internally, insufficient pressure went to the back wheels. The brake master cylinder is a two step cylinder.

Techno Duck · Techno Duck's Garage

Ive not had a chance to look at it myself yet, but Dan and i both think the problem is 'brake knockback' or hub flex. The rotor shimmys side to side slightly under hard cornering and spreads the brake pads and pistons out. This is why the pedal goes to the floor at that first push but comes right back to normal. Also why it only happens with turns. Its not totally uncommon for race cars.

We are just trying to figure out why its happening know. Some cars have it inherently (such as the bug eye WRX's) but i have never heard of it happening with a 944, even with the Brembo's.

GMingst · 03-12-2007, 08:51 AM

This is weird. Dan are the brake lines up front twisted badly? Really low pads will cause the caliper to pull back, but that happens straight, turning, whatever. I would like to see you repeat the test you did with both wheels off the ground supporting them by the control arms. That kind of takes the hub out of the mix. Try it and if the pedal doesn't drop then load one side (cinderblock), and repeat. maybe you can at least isolate the wheel where it is happening.

Graham

Jeeper31 · 03-12-2007, 09:36 AM

Quote:

Originally posted by GMingst
This is weird. Dan are the brake lines up front twisted badly? Really low pads will cause the caliper to pull back, but that happens straight, turning, whatever. I would like to see you repeat the test you did with both wheels off the ground supporting them by the control arms. That kind of takes the hub out of the mix. Try it and if the pedal doesn't drop then load one side (cinderblock), and repeat. maybe you can at least isolate the wheel where it is happening.

Graham

Hi Graham - Glad you popped up here. was looking forward to picking your brain at the autocross seminar but I hear you were out chasing red 944s.

Anyway, as far as the brake lines the PO put in steel lines so I don't think they are twisted. The pads seem to still have a good amount of meat on them. I will try the test again with the wheels suspended. I am predicting that it will not happen in this case.

I am hoping to settle this problem soon as I have an appointment with my mechanic for friday. I would love to solve this and save the $$.

rudidelange - this problem did not happen when I first got the car (December) but cropped up a few weeks ago. It's funny cause I always felt the brake pedal was too high to heel-toe and then one day I stepped down and it was level with the gas. I think i can rule out the back as when the car is stopped and I turn the wheels end to end it happens. Before turnign the wheels the brake level was fine. did it a few times and the pedal hight is always affected by turnign the front wheels.

nynor · 03-12-2007, 10:12 AM

bad master cylinder, is my bet.

GMingst · 03-12-2007, 11:51 AM

It absolutely could be the master, but what throw's me is it only is happening when the wheels are turned. Dan did a controlled test with the car stationary. When the wheels were turned lock to lock he could replicate the condition. The Master shouldn't care, but I have seen stranger things.

Techno Duck · Techno Duck's Garage

Graham, was nice to see you yesterday. At first i thought i was going to get held up by the mysterious biker in all black.

GMingst · 03-12-2007, 02:05 PM

Yeah I am morbid looking aren't I....

I am not big on dressing up like a Christmas Tree.. Must be the German in me..

Jeeper31 · 03-12-2007, 07:51 PM

ok. Jacked car up and turned wheel from end to end and the brake pedal did not sink too low. Tried what Graham said (sort of) by lowering one side of the car and turning the wheel. It seemd that with the car on this angle the brake pedal seemed low. It was hard to tell but i think it stayed low and I think the same thing happened on the other side. I am wondering if the angle of the car has anything to do with that.

Tomorrow I will lower the car and turn wheels with car on the ground, then check the pads to see if any have come away from the rotor.

toolboy62 · 03-13-2007, 07:20 PM

Have you tried simply running some fresh brake fluid through it? It doesn't *sound* like air in there... but couldn't hurt, anyway.

nate

Jeeper31 · 03-22-2007, 04:03 AM

Replaced the rotors and bled the system. Problem still happening. I raised one front wheel and turned the wheel end to end and the brake pedal was low on the first press. I tried the same thing again after raising the other front wheel and the brake pedal was low upon the first press. It seems like the brake pedal might be a bit spongier upon the first press. When I depress the pedal again it as solid.

I am running out of ideas. Is there any chance the spindle is worn and therefore causing the rotor to flex? Could I have gotten 2 bad inner bearings. They are adjusted correctly.