I was Slowly going down a hill and i started to lose brake pressure slowly but then i released the brake and pressure returned after a a pump or two. No Leaks externally on the truck that i can see. I Tried a mastercylinder thinking it was a internal leak with in blowing by the pressure seal. Bleed the brakes got normal brake pressure. Then i tried holding down the brake again while parked with no parking brake. I started to slowly lose pressure again but when released brake pressure returned ? I ve read that i should try adjusting the rear drum brakes but how would that help it?