Residual valve is a one way check valve. They are used inline with the brake lines to keep a certain pressure on the shoes or pads. In you case with all drums you need a 10 pounder (2pounds for disc). They are considered necessary for systems where the master cylinder can be below the brake cylinders. Fluid runs downhill so the fluid in your lines can run back into the master cylinder. They are usually located near the master cylinder. You will need one for all lines leaving the m.c.
They are around 1/2" in diameter and about 2" long with tube connections on the ends. Cost around 8 to 10 bucks.

Six volt guy living in a twelve volt world