Minimal rear brakes...Proportioning / combination valve fun and education.
 

We completed the 8 lug to 5 lug conversion on my 72 Suburban. Drop spindles up front and replacement rear axle from board member UNCLE. Looks great, sits right and now I can actually run a variety of wheels that don't suck!

Having had MANY trucks and had put brakes on all of them, I have NEVER dealt with a bad or "popped" proportioning or combination valve. This post is just to give others a little info on my experience as the research I did on the WEB was not reaL clear and NONE of the mentioned "fixes" worked for me.

For all you with disc/drum set ups, the first thing I will tell you is when playing with brakes, get that screw in centering tool that you put where the brake light sensor is in the prop valve. This keeps the rod in the valve where it needs to be.

My symptoms were very weak rear brakes. We had air free good flow to the rear and working front and rear on the rack but only the fronts would grab when stopping the weight of the truck. We had all new, hoses, rear lines, wheel cylinders, pads, calipers, drums, master cylinder...basically everything but the booster and prop valve. Booster check indicated good working order. After a few sets and aggravation I finally turned my attention to the combination valve. I noticed the end popped out and started reading about it. It seemed from research that this MUSt be the problem. Some said re-center the internal rod....mine only had a tiny hole under the sensor and you could not see the rod. We tried digging around in it with a pick but could not grab an edge to re-center. Others said open this brake line, open that brake line, slam the brakes, C clamps...you name it...we tried it all with no luck at ll. I had a few trucks I could have robbed one off of but the line locations were different so I just ordered one from Classic Industries and paid double for 2 day shipping. When my tracking info showed my 2 day shipping was actually 5+ days ???? I went on a mission in my shop to find one. The Rottenwood Garage ferry showed mercy on me and one appeared out of thin air and actually had the correct holes in all the right places.It was an aftermarket style but looked to be in good shape and it was not popped. I installed it along with the centering tool and let is sit over lunch to gravity bleed. Tightened it all up, re bled all 4 corners and wa-lah....rear brakes!!

If anyone else has additional information please post it.

Re: Minimal rear brakes...Proportioning / combination valve fun and education.
 

I'm actually in the middle of redoing my brake lines and converting to power brakes. My prop valve looks exactly like yours with that "pin" sticking out about 1/4". I've ordered the bleeder tool and its on its way. But what is the length suppose to be sticking out of the valve??

I took my sensor out of the top where the bleeder tool would install and I can see the brass components and the shuttle valve for lack of better terms and was able to move it back and forth with a flat head screwdriver, but as I did this that pin did not move and I can't seem to move it with pliers in or out...

Re: Minimal rear brakes...Proportioning / combination valve fun and education.
 

It is my understanding and from looking at all my others that it is actually SUPPOSED to be flush or slightly recessed when its in the correct position.

Re: Minimal rear brakes...Proportioning / combination valve fun and education.
 

This a cutaway btw just for good reference.

Re: Minimal rear brakes...Proportioning / combination valve fun and education.
 

That's so strange because based on the cutaway and when I took mine apart I don't even see how that pin moves...??

Re: Minimal rear brakes...Proportioning / combination valve fun and education.
 

The pin moves because of pressure differential. If one side in the cutaway loses pressure, the pin will move to that side by the combination of vacuum on the low pressure side and pressure on the side that is still functioning properly.

The pick method of reentering can be difficult, but have had pretty good luck doing it that way. The special tool would be helpful. The tool that threads into the sensor hole does a decent job of holding the differential switch centered while bleeding.