someone972

iTrader: (2)

I just got done installing an adjustable proportioning valve. This was to try and get the rear PBR disc swap to actually stop. Before the adjustable proportioning valve I was unable to even stop the rear wheels at idle when on jack stands. Now I can stop the wheels and even rev a bit without them slipping, so certainly an improvement. However I still cannot get the rears to lock up even on gravel. I have the adjustable proportioning valve at full pressure and it's still not locking. The next thing I'm going to try is ordering some Hawk HPS pads and get the rotors resurfaced; I suspect either the parts store pad is either glazed over or garbage. Other than that I'm out of ideas.

I have done up to this point:

• Replaced master cylinder
• New rubber lines all around
• Reman rear calipers
• Adjustable prop valve (plumbed through stock valve, removed spring in stock vavle).
• Bled the brakes to death.

I will likely remove the prop valve plunger and seal the plug instead of just removing the spring, but I doubt it will have a major effect on anything.


Anyone have any ideas why the rear brakes still aren't working at their best?

QwkTrip

iTrader: (22)

What prop valve are you using?

I had the SSBC and the rear brakes were worthless. Switched out to a Wilwood and it was a tremendous difference. The Wilwood can be adjusted up to 100% (no pressure reduction).

someone972

iTrader: (2)

I'm using an Allstar proportioning valve, this one:


It supposedly goes between full brake pressure and 57% reduction.

manualbrakes.com

I take it that your calipers are NON adjustable, but have you tried the free travel adjustment on the rear calipers?

https://www.thirdgen.org/forums/susp...ear-brake.html

I am not familiar with these calipers, but it sounds like your rear calipers are not able to build pressure because there is too much clearance between the pads and the rotor.

Usually for rear disc brakes, you don't need an adjustable valve because the caliper piston area is smaller and does not apply the same amount of clamping forces as the front calipers. Usually a adjustable prop valve is run fully open with almost all rear disc systems.

someone972

iTrader: (2)

I don't believe I have the adjustable calipers. Could very well be just that the rear piston is smaller, although I'd still feel better if I could get them to lock at all (just to know I've got them working to their best). We'll see if the name-brand pads make any difference, hopefully it gets rid of the annoying squeal these ones have too.

manualbrakes.com

In my opinion, you should hold off on $70 pads because even glazed over pads will lock up your tires on gravel if the rear calipers has enough clamping force.

Check if your bleeders are at the very top of the caliper. If they are not at the very top, they could hold air causing your weak clamping forces.

If your bleeders are in the correct orientation, you either have a bad master cylinder or the pads are to far away from the rotor.

I like to use the plug method to bleed master cylinders because it lets me know if the master cylinder can hold pressure. I do not like to take for granted that a NEW or rebuild master cylinder is good. The master cylinder is the most common issue when dealing with brake issues. I also like to use a syringe process as an extra step to bleed step bore master cylinders to make sure there is no air trapped in the residual valve for the front brakes (original front calipers on third gen f-bodies where low drag and required a step bore master cylinder to supply the extra volume needed for the low drag calipers to function correctly).

Steps to Bleed Step Bore Master Cylinder:
1. Mount master cylinder in a bench vise.
2. Take Brake Bleeder Syringe and fill with brake fluid making sure there is no air in the syringe
3. Force brake fluid through the rear outlet port of the master cylinder with the syringe. Do this step twice and plug the port with the plastic fitting supplied with the master cylinder
4. Force brake fluid through the front outlet port of the master cylinder with the syringe. Do this step twice then plug the port with the plastic fitting supplied with the master cylinder.
5. Fill the master cylinder reservoir with fluid.
6. Using a blunt end rod (NO phillips head or flat head screw drivers which could damage the piston or the bore) cycle the master cylinder piston until it becomes rock hard with almost NO movement. If there is excessive movement, the master cylinder is bad.
7. If the master cylinder piston becomes rock hard, hold pressure on the master cylinder piston for 45 seconds. If the master cylinder piston slowly sinks down the bore, you have a bad master cylinder. If the master cylinder holds pressure and does not sink down the bore, the master cylinder holds pressure and is good.

These steps will eliminate the master cylinder as an "issue" if you are having brake problems. If the master cylinder is holding pressure, then it will supply the pressure needed to clamp the rear caliper's pads against the rotors.

If the master cylinder tests good, the next issue is most likely excessive clearance between the brake pads and the rotor. If excessive clearance, there will need more volume of fluid to push the pads against the rotors and the master cylinder may not be able to supply the volume and build pressure in the rear lines. The caliper will bleed correctly, but the caliper never sees the line pressure it needs for proper clamping forces.

someone972

iTrader: (2)

Already ordered the pads, but it will give me an opportunity to address the annoying squeal I'm getting so no worries there. My bleeders are in the correct orientation, so shouldn't be holding excessive air. Thanks for the master cylinder bleed procedure. I bled it when I installed it, and it seemed OK, but it may need it again after fiddling with the brake lines since it's possible air got up into the bore. I had been using the trick where you keep the pedal depressed to keep it from draining itself when the lines were disconnected. There doesn't seem to be excessive clearance on the pads, but obviously I'm not a micrometer. It's possible one of the rotors is slightly warped though since I do feel a very slight pulse now sometimes. I'll have them turned when installing the pads and make sure there's no debris, should take care of that.

So my current plan of attack is as follows:
1. Remove prop valve plunger and seal the vented cap. More for peace of mind than expecting improvements.
2. Turn rear rotors, install new pads. Take the opportunity to do usual brake maintenance.
3. Bleed brakes again, follow usual bed in procedure for new pads, and see if it will lock them on gravel.
4. If it does not lock them on gravel, follow your master cylinder bleeding procedure.
5. If it still does not lock them on gravel, just deal with it and be happy they actually work now

In any case I'm still glad I got an adjustable valve installed. It will make it easy if I ever decide to upgrade the brakes to something with significantly more clamping force.