when i built my bike and bled the rear brakes they worked fine.then the next day no pressure.after bleeding and bleeding i gave up and bought another new master cylinder.i am going through it again.i bleed the brakes and get pressure and brake light works.check it hours later and they are week.with just pumping the pedal maybe 15or20 times i get it back.sounds to me there is a leak somewhere.checked all lines and they are tight at the fittings.both banjo washers are beat up do you think that could be the problem?this is driving me crazy,please help

 

Replace the washers and try bleeding them again. It's possible that you're pulling air into the system when you are letting off the brakes while pumping them