Do prop valves go bad?

[TugOwar]

Hi folks,

My 68 GMC 3/4 ton has the factory brake set up, including booster, MS and proportioning valve and I've thoroughly bled the lines.

When I press the brake pedal I have a good pedal but only have rear brakes to start with. Then the pedal will go on down and suddenly the front brakes will engage.

I'm wondering if the prop valve could be bad. Any ideas?

Thanks

Oh forgot to mention I HAVE adjusted the front brake shoes close to the drum.

[shuttermutt]

Yep, they can go bad and when they do, they'll do exactly what you describe. Double-check for leaks first, though.

[68C15]

adjust the rears. they come on first & until resistance to fluid pressure is attained the fronts do nothing. hope this makes sense.

[shuttermutt]

Originally Posted by 68C15

adjust the rears. they come on first & until resistance to fluid pressure is attained the fronts do nothing. hope this makes sense.

That's a good point and more likely than a bad valve.

[rage'nrat638]

well yes they do......BUTT......

my experence say's.....it's the master cyl......and might even be the push rod adjustment...between the master and the booster......

is your brake lite coming on????

assuming the rears are adjusted up...

just my experence......and my thought.......how am i

[Gordo]

Someone correct me if I'm wrong but, I always thought trucks with 4 wheel drum brakes (up to 70) used a distribution block and trucks with front disc brakes (71 and up) used a proportioning valve.

[TugOwar]

Ok I'll adjust the rears tomorrow and give her a try.

I have a very, very slight leak on the right front rubber hose but not enough to have this kind of effect.

The brake light was coming on weeks ago when I first bought it, drug it out of the weeds and fired it up, but went out as soon as I added brake fluid and bled the lines.

If I need a prop valve is that a parts store item or junk yard/online order item?

Thanks again for all the help.

[rage'nrat638]

my 3/4 ton has a pro-valve with a wire hooked to it.....and make's the light come on.....

someone else's question.....


junk yard......to get one.....

no leak in any brake system is a good thing....no matter how smalll


if you light was on and you added fluid....and is not on any more mean's the slide piston in the pro-valve is moving and working....

[TugOwar]

my 3/4 ton has a pro-valve with a wire hooked to it.....and make's the light come on.....

Same here, directly beneath and to the side of the master cylinder.

The chances of me finding a 67-72 in a junk yard to swipe this part from are virtually nil. Any other models that have the same valve or one that will work?

[Gordo]

That "pro valve" on a predisc brake truck is only a brake warning light switch. When a pressure differential occurs between the front and rear brake systems, the valve will shuttle toward the side with the low pressure and set off the brake light on the dash. The proportioning valve on a disc brake system provides balanced braking action between front and rear brakes under a wide range of braking conditions. The valve regulates the hydraulic pressure applied to the rear wheel cylinders, thus limiting rear braking action when high pressures are required at the front brakes.

[Rollie396]

Not to hijack this thread but....has anyone here relocated the provalve?That thing is oogly and I wondered if I could hide it on the frame or something.

[TugOwar]

So I don't have a proportioning valve then? And this brake warning light switch doesn't affect fluid flow, only senses it?

I don't think I've ever had a bad master cylinder produce these symptoms but I would think that would be the next likely culprit. Agree or disagree?

If I'm replacing the MS should I just go back with factory or is there an upgrade I should go to?

All thoughts/suggestions welcome. :GMC:

[rage'nrat638]

factory.......rebuilt from napa....about 13.00

just go for it......

[True Grit]

ok,since i'm having the near exact problem with my 71 , lets recapp. my brake pedal slowly goes to the floor , and eventually i have nothing, no brakes. but will do the same thing again next time. i know i have pressure to the rear cause it will lock up in wet grass or mud, but not the front. and will go to the floor if you hold pressure on it(the pedal).

[Gordo]

With a peddle that slowly (sometimes quickly) drops to the floor while under pressure you have a bad master cylinder. It has a "blow-by" situation, the seals are shot.

[True Grit]

thanks gordo, good thing i've still got the reciept, its only six months old.

[TugOwar]

New master cylinder installed today, replaced the leaking front brake hose, bled all lines. Decided to just pull all the drums and have a look. Actually looks pretty good, shoes about 1/2 worn, drums smooth and don't have a big lip on the edge. No leaking wheel cylinders, actually looks like things were gone through and freshened up not long ago.

Put everything back, adjusted all the brakes, bled everything again, took her for test drive, Still have the same BS!
Better pedal, quicker response, but still feels like I'm compressing air somewhere. I can hit the brake and feel the brakes start to grab but the pedal slowly goes down, hit the pedal again and everything tightens up, good pedal, then if I still have enough speed the rear brakes suddenly lock up and slide.

I can take the top off the MC and remove the bleeder from each front wheel and I have a good, fast flow of fluid.
I can do the same to the back and I get a flow of clean fluid but it's quite a bit slower flow compared to the front.

Based on this here is my theory: The rubber brake hose at the rear end is breaking up on the inside, like front ones do sometimes. I can imagine the inside of the hose folding up against pressure, then suddenly giving way and letting the fluid rush to the rear wheels. Maybe it could even hold air behind it because of restricted flow?

I don't know, I'm open for suggestions here guys. I don't expect perfect brakes from a 30+ y/o truck but I do expect a good, tight pedal and all four wheels to stop at the same rate. These big a** drums and wide shoes and power booster have the potential to provide great stopping power.