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!