I'd suspect the master cylinder. Severe angle or not, the piston bore is full of fluid at rest and will work fine for a single stroke. If the piston is dropping, somehow fluid is being bypassed or air in the lines being compressed. Considering this doesn't happen all the time, I suspect it's not air. Master cylinders are cheap enough to swap in a new one. There is a longer pressure holding demand on the system when you rely on the brakes to hold you on an incline as opposed to just stopping. Any leakage at the master cylinder would be more critical and show up. Still, this is just a guess.