[hibshman25]

A warped rotor will indeed cause the problem you describe. I've run into this many times on wrecked bikes. When sitting on bike lets say the right hand rotor has a low spot to the inside. Every time the low spot reaches the caliper pads it is going to compress the caliper pistons inward. When the low spot passess there is now a gap between the rotor and the pads. It now takes a few pumps with the lever to mate the pads up against the rotor again. The more pumps it takes to regain pressure the worse the rotor is bent.

 

[hibshman25]

Its certainly not going to be the master cylinder. Rolling the bike has no affect on a master cylinder. He stated that if he rolls the bike it loses pressure. If he stops and pumps it and it builds pressure the master cylinder is functioning fine. A visual form 5 ft away tells you nothing on rotor condition. Say for instance each rotor only has an 1/8th of an inch of runout. Combined, the master cylinder sees 1/4". That will take a few pumps to move the pistons against the rotors again and regain pressure.