Brake Proportioning Valve?

[Jforeman]

Has anyone ever installed a true proportioning valve?

My coach is stripped bare, and new floors installed. It will be a while before I start furnishing it. In the meantime, it's not very drivable because the rear wheels lock up super, stupid easily, It's also not very drivable because the rear wheel wells are out, but that problem is easy enough to solve...
I'd like an adjustable one, to keep up with the weight changes as the build progresses.


I also have an extremely fast idle that wasn't there before I had it smogged. I found one cap was pulled off a vacuum port but that didn't help. The brake makes a hissing sound too, and the cruise control hose is all bundled up and tied off under the floor. I don't know if that's normal or not. I also found a piece of foam insulation under the forward butterfly valve about the size of a marble. It starts pretty easily so I don't think there's any more debris? I'm pretty sure it got there when I had it smogged.

 

[RF_Burns]

I can see where a bare coach could have issues with the rears locking up. The OEM brake system did not reduce brake pressure to the rears, but if you have a brass "proportioning" valve, there is a good chance it limits the rear brake pressure to 700psi.
Wilwood, Speedway and other aftermarket brake suppliers offer adjustable "proportioning valves".

 

[billvv]

This brass replacement combination valve replaces the steel OEM valve for the GMC Motorhome. All other brass valves will proportion (reduce) pressure to the rear brakes.

https://www.gmcrvparts.com/ProductDetails.asp?ProductCode=GM4-PVMH

 

[pvfjr]

I found one cap was pulled off a vacuum port but that didn't help. The brake makes a hissing sound too,

You're on the right track, looking for missing caps. Try disconnecting and plugging the brake booster hose, and seeing if your idle improves. The one time I had a bad booster in a car, it did a lot of hissing/whooshing. The diaphragm had torn. This could definitely cause a decent vacuum leak.

 

[ScottC]

I remember I bought a new prop valve and had to modify it, The instructions must be somewhere on the web or possibly in my build thread. My brakes have the larger front calipers and stock drums on rear and they work great with the modded prop valve.

 

[Bhart]

In the GMC Vintage RVing magazine is an article by Dave Lenzi on modifying the proportioning valve, Winter 2021 Issue 154 page 24.

 

[Jforeman]

I should probably re-check my ride height, maybe if it's too high that will contribute.

I haven't started it yet but the PCV hose was a little loose and not seated fully on the barb.

 

[Richard RV]

In the GMC Vintage RVing magazine is an article by Dave Lenzi on modifying the proportioning valve, Winter 2021 Issue 154 page 24.

I ran across this in the PO's files the other day. Not a how to, but explains the operation. I really got a kick out of people telling Dave Lenzi he knows nothing about brakes!