65 Chevy C10 rear drum brake woes. please help.

[theastronaut]

If you have a combination valve, it has a hold off valve for the front lines that limits front brake application until line pressure builds to a certain point (I've read that it's roughly 40 psi but unsure of the actual number). This is to let the rear line pressure build enough to overcome the brake shoe return springs, so that the front and rear start building friction at the same time so the pads/shoes wear evenly and the front doesn't nosedive with only the front brakes working. Without the hold off valve the fronts would start working immediately since calipers have next to zero resistance (no return springs to overcome).

If there is a hold off valve, you should be getting rear flow/pressure immediately as long as you have the tool to lock the shuttle valve in place.

Getting the pedal ratio correct for boosted brakes will also help with pedal firmness. The stock ratio is 6.25 to 1, power brakes should have the pushrod point moved down to make the ratio 4 to 1. If you leave it 6.25 with a booster the pedal will go to the floor and make very little line pressure even when bled properly.