Open bleedvalve to check if there'e pressure in the system -when stepping on the brakepedal Look for gunk or "foreign-stuff" plus air-bubbles-if any bleed system.
Take offending side wheel and brakedrum off to see there's (enough)movement in the wheelcylider-pistons,as in my opinion they are the first thing to cock up when car is sitting still for any lenght of time.They are cheap and easy to locate so if in doubt get a new one
pump all brakefluid through and replace with fresh & waterfree stuff!
If car sits still, remember to pump the pedal once in a while to avoid it getting stuck............... -R.