I have 11.75 discs/2.75in bore GM calipers up front and stock 11x2.5 drums in the rear. I'm using Dr. Diff's aluminum MC, the original '67 drum/drum distribution block, and a rear prop valve - that's it. I've put in new rear wheel cylinders and gotten the air out of the system (believe me) including re-bleeding the MC.

Despite instructions otherwise that say the above is all I need, from what I've read it sounds like I really need a 10 psi residual pressure valve to get my rear drums to work right and firm up my pedal.

Right now, one direct stroke of the brakes = not that great, but 1 pump and the pedal comes up and it's better. More pumping doesn't do anything (used to, everything is bled out now). Turning my prop valve a few turns from all the way out makes the problem worse - I can feel the drums activate and the car slow more after a longer delay.

10 psi valve time for the rears? Is there any reason I wouldn't with my parts combo?

Call Cass and ask him if that master cylinder has the residual valve built into it I remove them from any master cylinder I use on disc brakes(that port only) when the master cylinder is higher than the disc brake calipers are, you need a 2 to 3 PSI if the master cylinder is lower than the disc brake calipers to prevent them draining back into the master cylinder. Let us know what you find out and do.

The alum. MC's don't have res.valves as far as I know.

Res. valves should only take away soms slack/spongyness out of the system, and not really solve a very deep pedal the first time the brakes are applied.

There's only a minimum of play between the brake pedal-rod and the MC?

Did you adjust the rear drums correctly (barely dragging against the drums when rotated by hand)?

If the slave cyls are large enough and the MC bore small enough I could see that it takes a fair amount of pedal travel to fill em. I would take off the MC lines & cap the MC with a pair of brass inverted flare male fittings from the Edelman cabinet at your parts house (they're cheap & you WILL reuse em) and if the MC is bled out & the cups are sealing excellent you will have a rock hard pedal with virtually NO travel. That gets the MC out of the way (assuming it's bore is large enough for your choice of slave cyls). Then I would hookup the line to the fronts & get them high and tight. Holler when you can with any news. To ans your Q maybe a residual valve in the rear line would solve this easily but sumpin dont sound right & I got 76 A body 2.75 discs/10" drums with the OE drum splitter and a truck 1&1/8" alum MC on a 65 dart (till it got totalled/not related to braking) & there's NO lag on the first pump