ZPmadA

I've owned my car for 2 years and the previous owner had a 928 bias valve installed to give more power to the rear brakes, close to 50/50. He said this modification helped him brake later while at the track. When I jack the car up and spin the wheels, the front wheels spin freely but the rears require some force to turn. I'm trying to figure out if this is caused by the 928 bias valve, or if my rear calipers are sticking and need rebuild. But then, what would the odds be of both my rear calipers needing rebuild, while the fronts being fine? Should i removed this 928 bias valve? I mainly use the car for commuting, with a few autocrosses thrown in.

V2Rocket

The bias valve seems to be pressurizing the rears even without the pedal down...you probably want to remove that.

Jfrahm

Um, wha? How 'bout less posting, more reading.

ninefiveone

This I gotta hear. Where did you come up with that diagnosis and solution?

V2Rocket

By applimificating an extraneous use of the scientific method to extrapolate from the supplied data a hypothesis.



Or just think about it. The cars don't usually do that, do they? Now with a 928 valve it does. Hmmm what could the cause POSSIBLY be.

And please tell me you checked the e-brake.

Riff

Well if you have the rear of the car up, I would think the rears would be harder to turn since you are spinning both sides (LSD test anyone?) vs simply spinning one wheel. That is not to say there is no drag in your rear caliper, but I do not think it due to the bias valve. I have a 33Bar bias valve in my car (I installed it) and the rears took the same effort to rotate before and after install.

HTH

pjburges

Unless the bias valve is broke or clogged in a weird way - it shouldnt hold pressure in the rear brake lines. You should check your EM brake and make sure it isnt dragging, before you pinpoint the bias valve as the perpatrator. Although if it's easy enough to take out - it certainly does get rid of one "hypothesis."

FRporscheman

A bias valve just sets the cross sectional area for hydraulic pressure going to the front, and to the rear brakes. When your foot is off the brake pedal, the valve isn't doing anything.

The real test to see if the brakes are dragging is to feel your wheel after driving for a while. If the wheel is much hotter than it should be, it means the brakes are dragging.

Oddjob

It wont be the rear proportioning valve. The 928 valve is the 5/33 bar valve. Absolutely fine to leave it on the car. No way the brake system can retain pressure after the pedal is released. Look elsewhere for your problem.

If the brakes are actually hanging up, check the calipers.

Perry 951

Crack a bleeder screw on the rear calipers; easy way to check if they are holding pressure.

ZPmadA

This idea came to my mind last night before going to bed. I was just so set on thinking it was the calipers that I didn't think of the lsd. Heres the main reason I think its the calipers. I have the typical squeeking 951 brakes which from what I hear is normal. However, when I get the brakes warmed up and I'm stuck in traffic, they sometimes squeek a little when I'm rolling with no brake pressure. Even when I'm coasting through town, and its kinda embarrasing. I can still roll at lights when they squeek so they can't be sticking too bad. I had a hard time depressing the pistons yesterday to install thicker/newer brake pads. Is there a tool anyone recommends? I tried using a large flathead screw driver but gave up because I was worried I would mess something up. It was requiring lots of force.

If I did this, what should I expect? Would I have to re-bleed the entire system after this test? Or just top off the brake fluid?

ehall

Have you checked the E brake yet? I haven't seen that question answered. This is common with a misadjusted e brake, or one that isn't fully disengaged due to...ummm...ahh... driver..um...error.

ZPmadA

I'm positive the e-brake was not on because I remember double checking the handle. I will check if its out of adjustment this afternoon while doing that caliper test.

Jfrahm

Seriously, please... you do _not_ need to post in every thread. If you care at all for this community do not post your uninformed guesses, just read what the experienced people have to say and learn something.

-Joel.

ehall

Jus an additional thought, how are the rear wheel bearings?