if the disc is obviously bent you will have to replace it, if the problem was never there till you hit the rock and the disc is obviously bent then it is likely the problem, what is happening when you lose brake pressure is that the brake is applied to the rotor it will grab on the high side then when it gets to the low side there is not enough fluid pressure built up to keep the pads in contact with the rotor, or the high side pushes the caliper piston in slightly then the low side comes around and there is not enough fluid in the system to react and take up the slack that was made when the high side pushed in the piston, therfore losing brake pressure, if you were to stop at this point and regrip the brake i would assume the 2nd or 3rd pull of the lever would get the piston out far enough to grab it again even if its on the low side, if the rotor is bent would be an obvious problem to start at, if for some reason it didnt fix the problem then your not out anything because you still needed a rotor, you should put your bike year and model in your signature so we know what machine we are talking about....
