What are symptoms of a bad prop. valve?

[rjfguitar]

I've had an ongoing brake issue with my K5. When I grab a foot full of brake I'll have both front and rear braking, and then on the very next push on the pedal I've got front brakes only, no backs. The brake light switch is also on, which I see that sensor is in the prop. valve.

I figured it was either the prop valve or master cylinder. I changed the master cylinder and that isn't the problem. It's basically got to be the prop. valve, I've bled the brakes a couple of times and get no air, just oil.

What exactly is inside a proprotioning valve? Are there springs and what not like a valve body in a trans? Just trying to think of what might be wrong with the prop. valve itself.

 

[George_Pimpdaddy]

hmm im sure you bench bled the master too right? not to sure where to find a new valve maybe dealership?

 

[jamber2541]

I had the same problem, if you bled the brakes several time time you need to keep bleeding until the problem goes away. Usually wont be on the same day. It took me about a month until they got right.

 

[muddybuddy]

definitely sounds like the prop valve cuz it regulates how much power going to front and rear. what u said bout it varying between both and just front brakes sounds like its bad. i believe the aftermarket prop valves are bout the same price as factory ones from the stealership so id go that route, if you have rear disc on ur 14b that might make them better as well.

if you know anybody with a vacuum bleeder that could make sure that there is no air in the system that would confirm that its the prop valve, or just an air pocket somewhere

 

[GM4wheeling]

If they're not too nasty and rusted up, you can crack the lines on the prop. valve and bleed it right there.

 

[dremu]

LMC has prop valves, to the tune of $100-150. Or the dealer. There are many, based on application, GVWR, phase of moon when the truck was manufactured, etc.

I'm generally convinced that proper bleeding is ALSO a function of the phase of the moon, and is only successful on prime-numbered Tuesdays. Also, both my K5's bleed perfectly well every time, and my crewcab is a freaking nightmare.

The voice of experience says ...

-- A

 

[JK5]

Yep....it's the valve...
I've had the same problems with a rig I had. Did you lose a rear brake line?
Since the valve rarely gets used, they will stick. I've taken them apart cleaned them if the bores weren't scored or rusty. Good Luck.

 

[GM4wheeling]

just did a google search and found them for $85... um, here..http://www.inlinetube.com/pro_valves.htm

 

[JBo214]

Agreed on the prop valve being the problem. I'm a GM tech and have seen these things bind up internally, as previously stated.

I have had lots of success "stabbing" the brake pedal and then bleeding it. This means having a partner to help. Have them really punch the brake pedal about 3-5 times and hold tight. Then you try bleeding the rear.

If you are just putting a new one on it though, it should take care of the problem.

If you do put a new one on, try a gravity bleed the system before you pump the pedal. Just open the rear screws and take off the master top. Keep fluid in the reservoir and let them bleed for a while, then do the pump method.

 

[johnnychimpo]

I built a pressure bleeder. tried to bleed my rears but got no fluid coming out I suspected the PV was bad.

 

[rjfguitar]

Well, then I'm going after the prop. valve then for sure. I think what I'll do is buy one from a wrecking yard, and see what happens.

Here's another question.. What is the difference between a 1/2ton and 3/4ton prop. valve? I'll exclude a 1ton valve as it could be very different due to the hydro boost setup..??..

I'm thinking I'd be better off with a 3/4 valve, as I have 3/4 brakes anyway..

 

[GM4wheeling]

Don't really know, but are the lines bigger?