Do I really need a residual pressure valve for rear drums.

[bc3j]

I have a manual disc/drum system (modeled after the disc o tech article) with a Dr Diff 15/16" aluminum master. After the car sits for several days, maybe as much as a week, the first push of the pedal gives me a slightly longer pedal travel that is still solid and at least 3" off the floor. Every other push has the pedal a little higher and it remains like that, solid with no settling lower. Is that due to the rear drum return springs pulling the shoes back over time? Or are the rear shoes slightly out of adjustment? My wheel cylinders are the new style with springs and the expanders. No leaks either. Right now, I'm not looking at this as a problem, but like some thoughts on it.  I pleased with the stopping power of the car.

[JS29]

Bleed it again, sometimes a little air gets trapped.   

[Chryco Psycho]

The factory master cylinders have a residual pressure valve built in so using a different master can require a RPV otherwise the seals in the wheel cylinders  can collapse & let air in

[bc3j]

I'll just install one. Believe 10# for the rear is good. I'll put it just before the tee on the axle. That'll be the easiest for me. I didn't originally put one in because I was under the impression that Mopar eliminated them around 1975 from the masters and used expanders in the wheel cylinders instead.  However, it sounds like one should be there.