Odd Brake Issue: pedal? system?..... input?

[Coley]

Ok...I have a very odd brake issue that I first started to notice last year.

First, some basic correct information:
-The truck is '72, 2wd, 1/2 ton with power brakes (discs on front)...all original.
-I have NO loss of brake fluid in the system...reservoir(s) are full.
-The brake pads and shoes are in good condition (+50% or more for sure)
Now:
-When I push the brake pedal down it hits a stopping point (midway?)....for about 5-10 seconds...then if pressure is continued (not lifted) it heads for the floor and the truck starts to creep.
Now, if I lift my foot and reapply pressure I get the pedal and the brakes back.

So...if the system was using fluid (losing it)...I might think I had air in the system, but there is no loss of fluid.

As well, the brakes are extremely 'grabby' when the truck is first started and moved...then they settle down within a minute or two.
After that I drive the truck all over the place, but I am aware that when I am at a light, I have to re-apply the brake pedal at least once to keep from creeping after initially stopping.
If I 'jam' the brakes while driving, etc.....they definitely grab.
so.....
Does this sound like the proportioning valve is working properly?
Is there a good way to analyze and check this?...a few intelligent, proven tests or?

Many Thanks!
Coley