Hi there. I used them on my '46 Chevy 2-ton. I upgraded to a dual-line master cylinder for a 1967 Chevy 1-ton with 4-wheel drums connected to a 7" dual-diaphragm booster run by a "universal" street-rod frame-mounted brake pedal but kept the stock 2-ton drums.
The residual valves were needed as even the "modern" 1-ton master didn't move enough brake fluid to expand the big wheel cylinders to stop the truck and the shoe return springs would push all the fluid out of the wheel cylinders and back to the master faster than I could pump the pedal.
The 10-lb valves hold enough pressure in the lines to keep the return springs from pushing all the fluid from the wheel cylinders without dragging the brakes.
Now, instead of the pedal going right to the floor without the truck slowing down (like it did without the residual valves), the pedal firms up after about an inch and a half of travel and will slide the tires in gravel with just half a pedal's worth of travel.
Pic below.
Brake booster, MC and pedal. 
