I have a 1989 IROC-Z with rear disc brakes and 9 bolt rear end. My rear disc brakes are both equally weak, while the front brakes are stong and lock up as they should. This makes an emissions inspection even more frustarting becasue the rear wheels will not stop spinning when the brakes are applied, inhibiting the test. I bled the brakes and they had no air in lines. I inspected the line with no apparent leaks or cracks. I also lubed the slides and all necesary parts. This helps the situation but they still don't seem strong enough. I do this every spring when I put the car on the road and I would like to fix this problem for good. I should mention that the e-brake works properly and holds strong. Is this a master cylider, proportioning valve problem, or should I rebuild or replace the calipers? What is the easiest way to completly flush all of the barke fluid? What is the best proportioning valve to use, without any major modifications? Please Help.

I am expierencing the same problem you are on my 85 IROC. I would like to hear what others have done for this problem before throwing a bunch of cash on it.

Do both of you guys have iron (Delco Moraine) calipers on the rear?

Yes, I beleive I do have iron calipers. They are the ones with the fins and are non-1LE.

If you have the iron calps, that's the problem. There was a recall for stick-shift (not a/t) vehicles with those iron calps. The darn things just stopped working. GM claimed it was because the owner's were failing to use the emergency brake. I used my emergency brake whenever I parked and I still had the problem.

There's a lot of info on this board about this. You may want to use the search function.

I haven't had the problem since I started using synthetic brake fluid.

The best solution: replace those calipers (have you changed the rear pads recently?) with PBRs. Dyno Don's holding a nine-bolt for me and I have the PBRs in the garage. Whenever I get enough money, the 10-bolt and iron calipers are history.

I simple question,

Does you emergency brake hold the car still from rolling when pulled?

If no, then your rear pistons are frozen in place from corrosion. Exchange the calipers for new replacements.

If Yes the E-brake still works to hold the car, then it is not your rear calipers (this is the more likely problem) It is in fact your Combintaion valve. This is very common for 3rd gens with the NON 1le prop valves to loose rear brake pressure over time.

I had a 4wheel disc single calipers all around and was using the 1LE prop valve 14089496 on it and the bias was great. This car stopped very quickly (the brakes were fully aftermarket goodies though with new stock calipers). The only reason why I sh*tcanned that brake system after all the money I putinto it was simply because the brake pads rattled in the calipers after about a weeks time no matter what I tried. The noise was embarassing. The heat I was putting into them would cause the tabs to soften and loosen on the pads and goop didn't help over time either.

My money in on your Combo valve being bad.

I agree with RTFC. My money would be on that combination valve as your problem. FYI, this is sometimes known as the proportioning valve and regulates the brake bias front/rear to make sure that the rear brakes get less brake fluid pressure than the fronts. You could also try pushing the rear caliper pistons back in the bore as if you replacing the brake pads. Reinstall the calipers and pull the hand brake or press the brake pedal to force the pistons back out to contact the brake pads. If the pistons compress and extend, then I doubt the calipers are the problem.

I also think we may have 2 different types of rear clipers in this post. I believe Kevin L. has the Delco Moraine cast iron calipers on his 85 IROC. I also have an 85 IROC and they are the Delco Moraines. The recall on the Delco Moraines was to address the failure of the hand brake to work, not really for weak rear calipers. The brake pads would fully retract into the caliper bore when brake pedal was released. As the pads wore, the hand brake could not develop enough travel to take up the slack so became useless.

PVV78 mentioned his calipers have fins. The Delco Moraines have no such fins so I think he has the PBRs.

Thanks for the info guys. It is a great help. My car does hold well with the E-brake applied. My next question may be a stupid one, but what exactly does PBR stand for? I don't think I have the delco moraines like which were mentioned. Thanks for all your help.

PBR is the company that designed the brake calipers. I think it stands for Performance Brake Resources or Performance Brake Research. I used to know because someone told me but I don't recall for sure.

PBR:

http://www.pbr.com.au/about/recognition.shtml


Ed

After changing the proportioning valve, will I need to bleed the master cylinder and all 4 calipers, or just the calipers?

I wouldn't start throwing parts at the car until you are sure that you have identified the problem.

It has to be the proportining valve. I do have the pbr calipers and not the ion calipers as you suggested. The e barke works and holds strong. The slides have been lubed and the pistons in the calipers are free. There is no air in any of the lines and there are no cracks or leaks. There is just no pressure at the rear discs which leads me to believe it is the valve. I'd rarther try that first then spend 3 times the money on the calipers if I don't need them. Thanks for your help.

If you have the PBRs, I think you are right.

After changing the proportioning valve, will I need to bleed the master cylinder and all 4 calipers, or just the calipers?

Yes I have the Delco Moraines calipers. E brake doesn't work for crap. Right side appears to work sort of and left don't for s***. I am not one to use used parts for this kind of problem but am trying to get this car safe for an 500 mile jaunt to THIRDGEN FEST. I have another set of calipers off my 9 bolt I could throw on. Am I unsane or realistic. I'm trying to bring a different car every year and don't want to bring the same one twice in a row!

Are the calps on the 9-bolt PBRs? If so, it is a common upgrade. Do a search and you'll find all kinds of information on it.

I have a 9bolt that has similar problems.
The parking brake won't hold and is adjusted as tight as it can be.
the rear calipers have been replaced with rebuilt ones but barely help stopping. I have taken it to several shops and they tell me the brakes have plenty of pressure to the rear when the brake pedal is pressed, and the e-brake is adjusted enough to "help" stop it. I have been trying to fix this problem for the last year with no luck. Everything has been replaced front to back in my brake system except the prop valve. I am thinking of installing a new prop valve or an adjustable prop valve to help even out the front and rear brake pressure.
Any advice or suggestions would be verry appriciated.

dusthow,
You didn't mention what kind of calipers you have. If they are the Delco Moraines (single piston cast iron caliper, no "cooling fins" on the caliper, uses the same brake pads as the front) you could try installing the recall kit. I'm not sure if 87's were affected by the failure of the rear calipers to adjust or not. That was what the recall kit addressed. The symptoms of this problem are parking brake will not hold, low brake pedal, and a noticeable gap between the rear brake pads and rotor. You will probably find out more about this by searching this site.

I'm a big fan of adjustable proportioning valves, expecially in F bodies where GM set the factory valve to give anemic rear brakes. Install an adjustable valve and disable the stock proportioning valve and you will have rear brakes.

I believe the only 9 bolt rears to be factory equipped with the "PBR" calipers are 1989.

As far as the 85 rears go, the auto trans cars had problems too. At least once between pad changes you had to remove the EB cable and tighten **** by hand. Then you could reattach the cable and the EB worked fine. Mine just decided to quit working altogether one day so I changed the calipers and they work much better..... for stock stuff. But that will be changing in July/August..... C4HD is coming.....

Just to let everyone know I put in the 1le proportining valve and the brakes work as they should. Thanks for all your help.

i have an 87 gta which originally had a drum rear. I put in an austrailian 9 rear with disc. Like everyone else the rear brakes suck. They don't stop on the emissions dyno....the rear wheels just keep spinning, unless you stand on the brakes, and then it barely stops. I think the autrailian rear is an '89, but i am not sure. What do you suggest? It seems as if I should change the prop, but I am not sure which one to get. An adjustable or a prop for an 89 disc car. Can anyone steer me in the right direction? I read about the prop mod where you put a 5/8" plug in there, but I am afraid that could cause the rears to lock and cause an accident. Which prop valve should I get? what part #. I have a prop in the garage that someone said was for an 89 rear disc car, but it doesn't look like there are part numbers on it. Will the adjustable prop bolt right in? Which adjustable one should I get? Will it throw codes? Is it easier to get the correct GM prop for that year? What part number would that be? thanks