I've currently got a problem on my '89 IROC where I simply cannot get any braking power from the rear. Even when I put the back on jackstands and put the transmission in drive with the engine at idle, flooring the brakes won't stop the rear tires from spinning (only mildly slows them down). Obviously, if the rears won't stop the tires with no load, they're going to do absolutely nothing against the momentum of the whole vehicle.

What's weird, however, is that the fronts are just fine. In fact, it really doesn't take much to get the front tires to squeak on this car (when braking)--but I'll bet that's because the fronts aren't getting any help from the rears.

So, I've been told that I need to look into a new proportioning valve. I'm really leaning towards an aftermarket adjustable valve (i.e. Wilwood). However, I'm still not completely clear on what it would actually do for me. My first thought is that, if the fronts can lock easily but the rears do nothing, then take some of the pressure from the front and send it to the back. However, I have heard that the proportioning valve can only cut pressure to two wheels. Obviously, I know the prop valve can't create pressure. But if all it can do is to cut pressure--and it looks like I don't have enough pressure in the back as it is--then what do I do?

BTW -- RE: getting the rears to work... I have already covered a lot of bases as far as that goes. New rear hoses (all three), new (rebuilt) calipers, new pads, new rotors. Plus, on the front, I rebuilt the calipers, have new pads, and new lines on the fronts as well.

Could somebody offer me some insight as to how a proportioning valve actually works so I can decide if that's what I need or not?

Thanks!

 

did you adjust the rears? sounds to simple but had to toss it out just in case..

the p valve does exactly what your saying it distrubits the presure to the front and rear of the car, you don;t usually have a 50/50 split. i my truck it even has a weight sensor that adjusts more rear for weight loads.

i would say your valve is shot if everything is adjusted properly

 

Hmm...

Well, let me see if my understanding of how the brakes actually work is correct. You've got the front brakes on one "isolated" system, and you've got the rears on another "isolated" system. The pressure for each system is generated in the master cylinder separately (that's why there is two lines comming out of the master cylinder: fronts and rears, right?), and each system goes through the same physical proportioning valve (but still remains separate within the valve).

I guess what I'm trying to figure out here is if it is correct to say that the fronts and rears are truly, completely isolated from each other--even though the lines might be running through the same components. If they are completely isolated, than the proportioning valve can only cut pressure to each individual line, but it CANNOT take pressure from one line and give it to the other. Is this correct?

I'm sure I need to look into a proportioning valve anyway, but another thing I'm starting to think is that I may also need a new master cylinder. Perhaps it's shot and not generating enough pressure for the rears to begin with.

Feel free to let me know if I need to go back to "Brakes 101" here...

Thanks for the reply!

 

ttt
I'd like to know also

 

A proportioning valve does what you desrcibed. It limits hydraulic pressure to a part of the brake system. A proportioning valve is usually installed so it limits pressure to the rear brakes since under braking weight shifts to the front of the car. The rear brakes cannot do as much work so the pressure must be limited to prevent the rear wheels from locking. If the rear wheels lock it causes wheel hop (rear wheels actually bounce on/off the pavement) or a spin out. Hence the name proportinong valve; the proportion of front/rear brake is controlled by the valve.

Here's some trivia many do not know. A proportioning valve changes the "proportion" with brake fluid pressure (at least a well designed unit works this way). This means that at light brake pedal pressure, the rear brakes have about the same fluid pressure as the fronts. The harder you push the brake pedal, the more the proportioning valve reduces the rear brake pressure. This is because at light brake pedal pressure there is little weight shift towards the front wheels and away from the rear. The rear brakes can safely do more work to stop the car. In a hard stop though, there is a lot of weight shifted to the front of the car, so the amount of rear brake has to be reduced.

OK, enough about what a proportioning valve does. Here are some ideas for you.

You may want to bleed the brakes again. There is a chance you still have some air in the rear brake lines that is reducing the rear brake effectiveness.

You may also want to try adjusting the rear brakes as JeffW mentioned. Replacing the rear calipers is fine but they must be adjusted to take up the space between the pads and rotors. Failure to do this usually causes a "spongey" or low brake pedal but it could be aggravating your no rear brake problem.

If all that fails to work you may want to try a new master cylinder or combination valve (technically, this is what the stock proportioning valve on our cars is called). Has you car always had this problem though? If so, it is not likely changing these parts will help. I believe GM intentionally dials down the rear brakes. Remember too much rear brake often locks the rear wheels causing a spin. This is unacceptable, so GM intentionally reduces the rear brake force substantially to make sure this never happens.

If you install an aftermarket adjustable proportioning valve you have control over when/how much the rear brake pressure is reduced. This is what the adjustable valve will do for you. I had the same problem on my car that you have now. If you install an adjustable valve, the key to making it really work is disabling the stock combination valve. You do this by removing the large hex nut on the end, removing the parts inside (spring, disc?, it's been a while since I gutted mine, don't remember) and reinstalling the hex nut. Now the full pressure is sent to the rear brakes and your adjustable proportioning valve is in charge.

By the way, I believe your description of the isolated front/rear brake systems is correct.