Originally Posted by
Red October
Master cylinders have an unfortunate habit of failing after repairs to the brake system which involve bleeding the system by pumping the pedal to move the fluid through the system. This is due to the fluid usually being neglected and not being changed regularly, as in annually, crud builds up on the walls of the master cylinder and can cause rust to form. When the piston travels further down the cylinder bore than usual the seal is exposed to the contaminated / damaged section damaging the seal.
The best way to bleed the system is to use a low pressure pump type bleeder. A UK company called Gunsons makes (or at least used to make) a home / handyman version which is basically a bottle filled with fresh fluid that attaches to the (filled) reservoir via a hose with a suitable adaptor and another hose that attaches to the air supply which is actually one of your tyres (!) drop the pressure to about 10psi and then bleed as usual (furthest bleeder first, nearest last). on drum brakes it's best to bleed with the adjusters backed off then adjust the brakes. The best way to check a master cylinder is to hold your foot gently on the pedal gradually increasing pressure, the pedal shouldn't drop.
Hope this helps.