The Stovebolt Site Forums The Shop Area Engine Shop propotioning/combination valve

I have a 1972 C-10 with front disc and rear drum stock brakes. They are manual brakes and everything is new (within the last year) except the combination valve. The problem is that there is no rear brakes at all. There is no pressure at the drum cylinders. The fronts still work though. From what I have read, the combo valve has everything to do with front and rear brake distribution. I figured that i might as well replace it since it is the only original brake part on the truck. One final piece of info, all this happens after it has been parked all winter. Please help if you can. Some new old stock would be nice.

i had this problem once with crushed brake line on the back of the rearend housing.

i had a simmilar problem it was that the brake hose had colapsed

There are other parts of the system that could be at fault, and those valves aren't cheap. If it were mine I'd look a little more before replacing it...

There are no crushed brake lines. A collapsed brake hose might be it though. I'll look into this but I'm ok with paying for a new valve. Thanks for the info guys, and let me know if anyone knows where to get one. Reproduction or NOS.

One thing i forgot to mention is that there was no pressure at the valve where the rear brake line goes in. If the line was crushed or obstructed, i would still have pedal, because there would be pressure buildup. Just not at the wheel cylinders. In this case, the pedal goes down with no resistance until the master cylinder piston gets to the front brake portion of the cylinder.

Sounds like your master went south. Try bleeding the system starting at the master, then the valve, then the wheels farthest from the master. If it bleeds out, you may get air again if it sits for long periods of time. This happened to me with a new master. I had to bleed every spring for a few years until the master or maybe the wheel cylinders "broke-in". I have'nt had a problem now for several years (except with the Corvette).
Good luck pinning it down,
Scott

48Big is correct. This almost *has* to be master-related, if you are not getting pressure or fluid to the inlet of the proportioning valve.

Do you get fluid flow out of the line from the master if you disconnect or loosen it at the prop valve? If you do, do you get fluid flow at the outlet side of the prop valve if you loosen that line? You might go to the FLAPS and get a couple of inverted flare plugs and try isolating things. If you plug the outlet of the master cylinder, you should immediately get a high, firm pedal. If you do, the trouble is further down the line. If you don't, the trouble is in the master. Same thing applies if you plug the outlet of the prop valve.

Keep in mind that the only place in the system where fluid can move without leaking is in the master cylinder. If fluid moves anywhere else in the system without doing work, it has to leak out. (Well...you can have a case where the master and the prop valve fail in just the right ways at the same time. If this happens fluid will pump from one compartment of the reservoir to the other.)

Hmmm....you say, "...all this happens after it has been parked all winter..." Is this a repeating problem, year after year, and if so what did you do to fix it in the past? If you were able to correct it by bleeding the rear brakes, then it may be the proportioning valve. One of the functions in a drum/disc combination valve is the residual valve for the rear wheel cylinders. The function of the residual valve is to keep air from pumping past the seals into the wheel cylinders, so if the valve is not working you can get air into the cylinders. In that case it would be time to replace the combo valve.

The Truck Shop in Orange, CA (800-243-8947)has them. $50 for a GM replacement, $80 for one that looks like it might be an OE style.

Joe

Follow Joe's instructions to check things. Also, you need to flush your brake system. The industry recommends annually, I recommend every other year for most vehicles. Regular brake fluid absorbs water and is easily contaminated, probably why you have a problem.

If the truck sets all winter every year, you might want to convert to DOT 5 fluid, silicone base. It doesn't absorb water, but is a little tougher to bleed and not quite as responsive as DOT 3. Biggest issue with switching is you have to replace all rubber in the system, hoses and cylinders.

The proportioning valves are sold by some of the vendors. Bought mine from GMC Pauls, but there are others. Go to the links section and look around.