Weird brakes?

[sixty8chev]

Ok heres the problem
My brakes work well 90% of the time, as well as 4 wheel drums can, but about 2 or 3 times a day the pedal will go slowly to the floor, it comes back up then works fine again.
Its not losing any fluid and I replaced the rear wheel cylinders last month and the fronts in the summer, new shoes at the same times, andits probably been bled about 20 times since to try and get rid of the problem.

SO I'm gonna go out and grab a new master cylinder this morning, but just in case that doesn't do it or if somebody has a suggestion that can save me the drive I would really like to hear it.