GMC Combination Valves

The original brake combinaton valve installed in our coaches peforms two functions: it activates a warning light in the dash if there is unequal pressure between the front and rear brakes such as if there's a fluid leak on one side, and it includes a metering valve to delay the front disc brakes so they don't engage before the slow rear drum brakes.

Once you go to an all disc system, you no longer need the metering valve function. While a lot of folks have simply left it in place, it does appear to inhibit the front brakes even if it's working correctly. If you measure temperatures after braking, you'll probably find the front discs stay much cooler than the rear discs with the metering valve installed. Fairly often, the metering spring mechanism is full of gunk or rust, and unable to work properly, and that will certainly mess with the brake balance, maybe even severely.

If you want to replace the combination valve, the correct replacement is a universal type PV4 for use with all-disc brakes. The problem though, is you'll have to remove the brake lines from the old unit, and that can be really difficult. If you are replacing the brake lines, it's probably easiest to replace the valve.

You can remove the metering function from the combination valve while it is still installed on the coach. Based on the excellent pictures from Ken Henderson, that's what I did. (See Ken's pictures here: http://www.gmcmhphotos.com/photos/deciphering-the-combination-valve/p5314.html)>

The combination valve is located on the left side of the coach at the back of the front wheel well, mounted on the frame. You'll need a pan to catch brake fluid when you open the valve. To minimize fluid loss, you can disconnect the small (front) brake line from the master cylinder and install a plug before opening the combination valve. Only the front brake lines should be affected.

Unscrew the cap from the end of the valve body. I tried using a fixed wrench, but was unable to budge it without flexing the mounting bracket and scarring up the nut. A single carefully aligned bump with an impact socket broke it loose. The nut is really thin, so it's a little hard to get a wrench or socket on it, and it's easily damaged.

Remove the cap and pull out the metering valve piston, springs, etc. The little top hat and spring may fall off the back, so be sure to fish those out too. There should be nothing left inside this half of the combination valve.

Pull the rubber cap off, and remove the metering valve assembly from the cap.

Thread the hole left in the cap where the metering valve pin used to be. I used an 8-32 tap, but Ken mentioned 10-32, so maybe there are different size holes. I did not have to drill it first.

Coat the threads of a suitable machine screw with epoxy and/or teflon tape to seal the threads, and screw this into the threaded hole in the cap to seal it. Check the o-ring on the base of the cap is in good condition and replace if needed. Apply teflon tape to the cap threads if desired, but be sure to keep the tape back from the last couple of threads so it doesn't end up in the brake fluid.

Reinstall the cap on the combination valve body. I couldn't find a torque value for this, but it shouldn't be too much since it has an o-ring to form the seal.

Check for leaks after you bleed the system.

Copyright © 2013 by K. Bradley