brake proportioning valve on drum/drum setups

[jocko]

Good evening truckers. Quick question on a subj that came up recently. I've been planning a stock drum/drum upgrade that adds a 67 C10 dual res master cylinder (for safety) and a 75 impala booster (for laziness). I've got CaptainFab's great looking firewall mount for the booster - but one question has come up that's got me a little stumped.

ChiefRocka did a GREAT thread a while back about splitting the lines, i.e. just installing a dual res mc into a stock drum/drum setup and nothing more (i.e. no disks, no booster, etc, just safety upgrade to dual res mc).

My question is this - if I understand it correctly, there should be no need for a proportioning valve on a dual res master cylinder (and the drum/drum master has same size reservoirs). Chief's great thread mentions this in one of the answers to one of the posted questions. It makes sense to me, BUT - when I look at stock dual res master cylinder 67 C10 setups - there is a block that connects the two circuits (just outside the master cylinder, right where you'd see a modern prop valve...) and I am not sure what it is. It's not a prop valve. Is it only a pressure sensor the detects a single circuit failure and warns you so you know that once you get home (on the remaining curcuit, i.e. the whole point of a dual res mc) to check out what the problem is - you're on the "old" system of 1 circuit at that point and only providing hyd pressure to the front or rear brake sys.

So - question #1: what IS that silly little block for? (in other words, if I don't care about having a failed circuit sensed, is there any reason for that block? In my pea brain, if all I want is a separate circuit for the front and rear brakes, why ever cross the lines at all?

Then, I came across another thought (aka question #2) - the front wheel cylinders are 1-1/8" dia while the rears are 1" dia. So, does that mean there DOES need to be some sort of prop valve to help balance the braking system? (OR, is the balance actually built in to make the system bias to the front by putting the larger dia wheel cylinders on the front wheels?)

The more I think about it, the more my head hurts?
so, in a nutshell -
#1 what is the little valve/check ball/pressure sensor/thingy that connects the lines from the 2 reservoirs just outside the master cylinder on DRUM/DRUM setups? (It "looks" like a prop valve, but it isn't, I don't think....)
#2 is a prop valve needed or not in a drum/drum setup with a dual reservoir master cylinder?

Thanks for your time!

[markeb01]

I’ve never heard of a drum/drum system needing a proportioning valve. Any system bias is built into the design by the size of the wheel cylinders. Since modern prop valves are actually referred to as “combination” valves (proportioning and warning switch) I would assume what you are asking about is a circuit failure warning switch. It is a simple ball check valve that moves off center activating a warning light if pressure on both sides is not equal. Even though I have a 1975 prop valve on mine, I’ve never connected a warning light to the switch. If one system fails it should be pretty obvious in the way the truck stops with only two wheels doing all the work.

[jocko]

markeb01, that is EXACTLY what I needed to know. Thanks very much!