Manual brakes don't have a prop valve.
There is a switch in the distribution block at the junction of the two master lines feeding the split to the front / rear. This is a piston operated deal and some of these when the piston gets "off center" can shut off one end of the car. This is usually obvious, because you won't have one end or the other, IE no front or no rear.
If you have no rear this is sometimes harder to detect, without "dynamiting" the brakes on gravel ETC, IE locking them up.
"What causes" drums brakes to be hard and stiff..............in no order
too large master bore, causes more pedal pressure necessary
hard, glazed linings caused by lots and lots of hard high speed braking
sometimes the TYPE of lining is way off. Way way way back "then" you could get metallic high performance lining, "Velvatouch" was one. This stuff was HORRID. They'd cool down from non-use on a long drive, and you'd need to make a hard stop. They would NOT stop cold!!! You'd tromp on the pedal HARD and they wouldn't do SQUAT. THEN they'd start to warm up and STOP!!! and you'd have to "back off" the pedal to prevent lockup.
drums that have been turned "too large" which causes the shoes to only contact the drums in the "center" of the lining. Think of two circles fitting inside one another. If you make the outer one larger, IE "turning" the drums or wearing them, then the inner circle becomes relatively smaller.
worn drums that are grooved, worn uneven, and have then had new shoes installed, which won't fit into these imperfections (grooves) and therefore become "glazed" on the high spots.
Huge oversized (diameter) tires, a common problem on pickups. The larger diameter the tire, the more difficult it is to stop.
