“A residual valve is usually warranted in the rear brake line when disc/drum arrangement is in place, just as the necessity of having a Proportioning valve. Shoes, by virtue of their design, "rest" a greater distance from the drum surface compared to disc. The residual valve acts to prevent the brake shoe from retracting from the drum this "greater distance" by maintaining a small bit of pressure in the rear line, thus, preventing the shoes from retracting more as they would without it”
NOT picking on you......
This statement is a total fallacy.
Neither a 2# or a 10# residual valve will do ANYTHING to battle drum shoe tension. Those springs are way stronger.
A residual valve is plumbed in line to keep pressure in the line, so that fluid will have a far lesser tendency to flow back into the master cylinder.
Also a big deal in the street rod business where brake calipers sometime wind up being higher in the brake system than master cylinders.
ex-Global West GM
As the drill sergeant said, "I taught you everything you know. I didn't teach you everything I know."
"Human beings, who are almost unique in having the ability to learn from the experience of others, are also remarkable for their apparent disinclination to do so."
- Douglas Adams