proportioning valves- What makes em tick

I have a couple of questions about proportioning valves that I have been pondering.
It started with an issue on a 78 CJ5 I bought last year that I have been working on. After I drive it a while the brakes seem to be holding, like the proportioning valve isn't allowing the brake fluid pressure to bleed back off, and keeping the calipers clamped. If I open the bleeders for the front discs momentarily, it relieves the pressure and works great again for a hundred miles or so. I assume it is the proportioning valve causing the problem.
So, my first question is, what is inside the valve that could cause this (how does it work)? Can it be cleaned; blow compressed air through it and it will be OK? Or would that toast some delicate mechanism inside?
2nd, my manual says that to bleed the brakes, you have to depress a button on the valve. This doesn’t make sense to me, as the fluid has to pass through it all the time, or the brakes wouldn't work. So what’s the point of holding the button in?
Can anyone shed some light on these mysteries?
Thanks.

 

Sounds like a residual pressure valve. Have you gone to disk brakes? If so you need to remove the residual pressure valve and put in a 2lb unit. Drum breaks use a 10lb unit.

 

Stock disc brakes on the front drums on back, stock master cylinder.

 

Have you got the brake lines on the MC hooked up right? The front chamber goes to the rear and is much smaller than the rear chamber that goes to the front brakes this is because as the pads on the front wear, extra fluid is needed to bake up the space behind the piston.


Anyway here is a "Combination valve" as it has a 2 stage propotioning valve, safety valve and front metering section. The safety valve moves back and forth to cut off the sections of the brakes that have lost pressure. This allows the remaing section of brakes to still operate.

The proportioning system in the rear is two staged to ive you different braking characteristics under a normal braking situation and an emergency situation.
Also since the rear shoes pull away from the drum, the combination valve has to delay the front brakes until the rear engage. Then you need less pressure for drum brakes then you do for disks and the rear has less weight during braking. so you have to cut down the amount of pressure going to the rear by a certian "proportion" Thus the name.

There is a tool for pushing or pulling the "pin"

 

Good info John. It looks like the brake lines are routed correctly to the MC. But as I look at it again, I'm wondering if the PO put the correct MC on. It looks fairly new. Jeff may be on to something, I assumed it was stock. Is there any way to tell if it is the MC is for front drums or discs? I think one was standard and the other optional on the 78s.

 

IF both resivors are the same size then it's for drums/drums if one is bigger than the other then it's for disk/drums

 

that seems a little overboard. my jeep sits for 4 or 5 months during the winter, but i surely don't replace the entire system every spring.

 

All of the brake lines are less than 5 years old, per the PO. While its possible, I really don't think they are the problem.
Do the valves themselves go bad very often?