The residual pressure valve [thought I would throw in another name] is a check valve that keeps a bit of pressure/fluid in the rear brake line. I think it's about 2-5 psi but don't really remember. As the drum wears the shoes have to travel further to hit the drums which in turn takes more fluid. The valve reduces the amount of fluid needed to apply the rear brakes. Four wheel drums have a check valve in the master cylinder but disc brakes do not use the valve thus a separate one for the rear brakes.

Here's a thought, If your rear brakes are way out of adjustment they could be taking more fluid than the M/C can give in a single application causing you to pump the brakes to get enough fluid to get pressure?

Another M/C check - pump the brakes untill you get good pressure then just hold the pedal down if it [the pedal] slowly moves to the floor you have an internal leak in the M/C.