72 C20 prop valve/brake light switch problem

[jocko]

Quick question for you brake folk - in a 72, the prop valve is actually a "prop" valve, unlike pre-71's (correct?). But it also serves the function of a brake hyd pressure loss switch (correct?)

Does it function as an isolation valve so you only lose the fluid from the single circuit and as a signal to tell you that's happened - or - does it serve ONLY as a signal to let you know that either the front or rear circuit is out?

So, if the light stays on after the initial startup and as you're driving down the road, I'm assuming that means I'm on only one circuit. Yet the truck stopped perfectly - no rear lockup, no swerve, no dive.

Thoughts?

Getting ready to take this thing on about a 250 mile haul (just to get it home) and wondering if it could be JUST the switch being bad itself (i.e. because the brakes "seem" just fine - although I didn't do any panic stop tests in it...) or if it definitely means I'm on the front or rears only.

Lastly - resetting it. This truck has been sitting for some time. So, quite possible it bled down/leaked a bit and lost pressure on one side that was sensed right at startup. So, wondering if, since it's stopping ok, that it can be reset to see if it comes back.

What does resetting the switch (once someone descibes "how" and hopefully with a pic or two! ) actually do? Does it just reset the electronics - or does it physically open the proportioning capability again?

I think I can wiggle my way thru this, but will only have a little time to tinker before I get on the road with this old truck - so thought I'd ask for some input from those that have worked on this pressure diff switch/prop valve before. Pls keep in mind, really only talking about the 71/72 switches since pre-71's were pressure differential signal only (no prop capability, and none needed) if I'm not mistaken.

Thanks!

[SS Tim]

There is a spool inside the valve that moves off center grounding the brake lamp circuit when it senses a pressure differential. It could be a M/C problem not providing equal pressure to the valve. Also could be the result of improper bleeding allowing the spool to move. The easiest thing to check first is disconnect the wire and the switch and see if the light stays on. That would be a wire issue. Normally a couple of hard stomps on the brakes will center up the valve if everything is in order. IIRC we went over this with Billy on his C30 not too long ago. Check out pages 5-2 thru 5-4 in the 72 chassis service manual.

[cdowns]

have u chacked brake fluid level// have you checked the condition of brake pads and shoes?

[Stocker]

Good info in the replies above. There's no electronics involved, just the switch applying a ground to the wire. If fluids are good and no bleeding is needed, try pressing hard on the brake pedal a few times to reset the switch.

[jocko]

cdowns - fluid level and consumables are fine - as mentioned, stops very smoothly with plenty of stopping power.

Thx Tim & stocker - will give it a stomp and check the wire. Didn't have the opportunity to yank the wire at the time, was a quick look with the po at that point. mostly prepping for "the return".

[jocko]

thx all - when I picked er up, the rear brake reservoir (i.e. the fwd one) was empty. Filled it up and crossed my fingers to get it home. Didn't see any visible leaks, but am sure there's air in the system. Pounding the pedal didn't help - most likely because of the air. Haven't had a chance to tear it apart yet, but there you go - the switch actually worked correctly to indicate a pressure differential. Good to know!

I'll bleed em and check the calipers and wheel cyls for leaks, and go from there. Thanks again for all the help.

[MagmaJct]

Well, how'd it go? My "brake" light came on recently. The rear reservoir ran dry. I filled it and bled the brakes. Light now goes dark when I start the engine. I didn't have to do any special procedure.

I will monitor the level at well to determine if there are any leaks. I've learned that leaks can be tricky. If it's a leaky wheel cylinder, the fluid will burn off before you ever see any leaks.

[jocko]

Well, I just did this on the 71 (both trucks had the light on...) and it worked fine. Once I got a sealed system and pressure on both sides - heard a loud pop as the prop valve reset itself and the light went out with the key on. It comes on at startup only, as it should, and extinguishes immediately thereafter. Whew... So, the 71 has an all new, top to bottom, lines, calipers, drums, rotors, EVERYthing brake system and it's all bled out and ready to roll. As soon as I can get some more tires since the ones I had set aside are now on the 72... But, the hard part is done, the rest is gravy.

Now, for the 72, gonna pop the rear drums and see what's leaking and, if nothing, just bleed it well and see if I can get the light to go out with a few good stomps.

[SS Tim]

Sounds like progress to me. That always feels good.