Could be movement in the bottom pivot pin or the drums have that much wear that it take 2 pumps to push the shoes out, i sorted this by cutting a piece of brass pipe and fitted it over the spring pin on the shoe then the spring goes back on to hold it in place (makes the pin bigger giving enough to adjust on the snail) if that makes sense.
It's usually always adjustment with these brakes, even when it feels like the master cylinder.
i only know this after changing most of the system in the hope of fixing the problem.
Make sure the pedal is at its correct height and all brake shoes are correctly adjusted before you write off the master cylinder.
Britpart brake parts are not worth the money and hassle (shoes delaminate and cylinders leak )
hope this helps
ta willie