renizzi

About two weeks ago the brake pressure went out on my car when i bled the brakes no fluid would come out of the valves except the front left the others it would just trickle out but there was fluid in the resevoir. I was told the master cylinder was bad so my cousin took it out and said it had black gunk in the bottom of it he cleaned it out and put it back in. I watched him as he did this he was pressing the piston and fluid would squirt out so he assured me that it was still good. Two weeks later the pressure goes out again the fluid is still at a good level but i have to floor the brake to stop. after 2 days of having no pressure at all im driving and all of a sudden the pressure comes back and the car seems to be braking fine. any suggestions