I know what a hold-off valve does. I never mentioned it's operation.
Correct. I mentioned it as further proof that the RPV doesn't keep the shoes near the drums.
I also never stated the RPV was used to adjust shoes.
An RPV maintains 8-16 psi in the rear drum system to overcome the shoe pull-back springs and allows the shoes to stay out close to the drums for instant response to fluid pressure.
What is that if it isn't adjusting the shoe-to-drum clearance?
If there is line pressure of 8-10 PSI between the check valve and brake wheel cylinder cups, isn't that pressure in opposition to spring pressure?
Yes, it opposes spring pressure (tension). You therefore have a situation where ~16 psi opposes ~50+ psi. Which results in the shoes coming to rest on the anchor pin unless there's something wrong with the brake mechanism such as an over-tensioned or seized park-brake cable; or the shoes are hung-up on the grooves worn into the brake backing-plate.
I also did not think air was supposed to enter a wheel cylinder if the components were adjusted properly and in good condition.
Yes. Air "shouldn't" enter the system. Apparently, with the master under the floor (siphon effect-fluid over-fills the reservoir) or on the firewall but with marginal wheel cylinder cups, air DOES get into the system past the wheel cylinder seals. This might be made worse if the "venting" described in the Delco manual--where fluid from the reservoir bypasses the master cylinder cup(s) under momentary vacuum--isn't effective; perhaps because it's -30 below and the rubber cups are stiff and the fluid is cold and thick.
I was under the impression his information was correct due to his background. Perhaps any info posted should be regarded as 100% false unless completely validated by numerous sources. I generally never believe anything I read. In John's case I assumed it was correct. It would seem best to doubt everyone as a precautionary measure. Thanks for posting the copy of the manual.
Life's good. We're OK. Happy to put this issue to bed.
What thread? I would not think an RPV would be moved from an MC to a combination valve.
Doing a quick search. I read about RPVs in the combo valve fairly recently, but this thread is many years old. It's not the one I remember. Post #8.
10 lb residual valve for drums????
Even older--Post #2
Change the Proportioning Valve?
From 2010--Post #3
Running brake lines. what do i need?
None of these posts is the one I "sort of" remember reading recently--but--they all claim that there's an RPV somewhere else in the brake system.
I'll continue to look...
This is interesting. Post 2--cup expanders. Completely forgot about the spring windings!
Can I use Disc/drum master and booster on drum/drum at the wheels
[Edit 2] I give up. I can't find a recent post about RPVs in the combo valve. Maybe it was on another forum.