Why don't my rear brakes work?

[fronton]

Another test drive today. 70% of the time it feels like all brakes are working and it is great, strong crisp braking experience regardless of the speed. 30% of the time if I hit the brakes hard it feels a little dull and the pedal is hard. Still rust spots on the calipers so they have not worked once since the ABS delete, new MC and new booster and 5 bleedings.

Any more ideas??

[AndrewBird]

Have you pulled the rear calipers and checked to see if they are seized? How does your rear rubber brake line look? Then can collapse internally and not allow fluid to pass through.

[fronton]

Have you pulled the rear calipers and checked to see if they are seized? How does your rear rubber brake line look? Then can collapse internally and not allow fluid to pass through.

rear rubber brake line looks good, flows as easy through the rears and fronts when bleeding. I haven't pulled the calipers but not sure why they would seize at the same time. Is there a way to try to unstick them while they are still on if they are stuck open?

[jlevie]

The only reason the calipers would stick open is if they aren't getting full pressure from the master cylinder. With both calipers doing this (and ABS not present) a lack of pressure has to be the master cylinder, bias valve, or the line running to the back of the car. At the back the line splits to each wheel, so a problem from that point on normally affects just one wheel.

[fronton]

Thanks, my next toubleshoot will be the bias valve. This issue was going on so I bought a brand new MC which did nothing. So I think I have two good MCs now but the issue is elsewhere.

If the bias valve is bad though, wouldn't that mean I couldn't pressure bleed the rears either?

[Ceeker]

are you sure you don't have a vacuum leak somewhere that is intermittent? A loss of vacuum to the booster will cause a hard pedal. check the line going into the booster.

[jlevie]

Thanks, my next toubleshoot will be the bias valve. This issue was going on so I bought a brand new MC which did nothing. So I think I have two good MCs now but the issue is elsewhere.

If the bias valve is bad though, wouldn't that mean I couldn't pressure bleed the rears either?

The bias valve is pressure regulator. It could be bad and limiting pressure to the rear brakes but still flow fluid when the line pressure is low. That would not keep you from bleeding the brakes, but it would reduce the pressure on the rear calipers.

[fronton]

The bias valve is pressure regulator. It could be bad and limiting pressure to the rear brakes but still flow fluid when the line pressure is low. That would not keep you from bleeding the brakes, but it would reduce the pressure on the rear calipers.

Good to know. I doubt it is the pressure valve mostly because it would be weird coincidence to have that go bad at the same time I deleted my ABS and put in a M5 booster and MC. I will probably put a new one in though, just to be sure. Sucks they are $70!

Would I be better off 'upgrading' to an adjustable prop valve such as: http://www.summitracing.com/parts/WIL-260-8419/

[jlevie]

With ABS deleted, an adjustable bias valve would be a good idea. You'll have to adjust it, but once set you shouldn't have to fiddle with it again.

[fronton]

I just ordered new rear lines (inner and outer), a new BMW prop valve, and rear caliper rebuild kits. One of these better fix it.

[fronton]

Okay so when I took my rear calipers off to reseal them, I found that the inside of the rotors was not spot rusting like the outside. This tells me that they are not getting enough pressure to clamp the outside of the rotor as well as the inside. The rotors are within spec thickness (12mm). The slide pins are clean smooth and I got tons of red/brown gunk out of the calipers. However, the rears are still not fully engaging.

I am going to replace the pressure bias valve next but curious, if the new discovery of the rotors clearly being squeezed on the inside only means anything specific to anyone. Thanks!

[jlevie]

The only thing other than lack of hydraulic pressure that would cause only the inside pad to contact the rotors would be if the calipers were binding on the carriers, which is very unlikely.

[fronton]

The only thing other than lack of hydraulic pressure that would cause only the inside pad to contact the rotors would be if the calipers were binding on the carriers, which is very unlikely.

I replaced the black rubber slider housings just to be sure but both the pins and the housing were already clean and sliding smoothly. I also replaced the calipers' soft line but it didn't appear to be in letting the system down either.

So it is down to the bias valve only right? Or could it also be yet another faulty MC?

[Todd Black 88]

Find the issue? I have the same thing, no rear brakes.

[jlevie]

I replaced the black rubber slider housings just to be sure but both the pins and the housing were already clean and sliding smoothly. I also replaced the calipers' soft line but it didn't appear to be in letting the system down either.

So it is down to the bias valve only right? Or could it also be yet another faulty MC?

What replacement master cylinder are you using? Or perhaps more to the point if it wasn't a new part costing $200-300, the quality and operation are suspect.