when i bought my truck the po had put 3 master cylenders on it prior to me buying, because of the same issues. that was 2 remans and last one was a brand new one and still had the same issues. i took the new one off cleaned it up internaly made sure all the piston and seals were alright, installed it, and bled all the lines several times with a vacumn bleeder. there were no air in the lines what so ever, but still had the bleeding pedal. there were two things that cured everything up, the rabs valve and the rear brakes were out of adjustment. the first thing i did was checked my emergency brake pedal to see how far it pushed down and it went almost all the way to the floor, so i knew they were out of adjustment really bad. i adjusted them the way they were supposed to be and brakes were alot better, but still slowly went to the floor. second thing i did waqs took the spring out of the rabs valve and put the nut back on it and drove the truck around the block constantly smashing the brakes. i dont recomend leaving it out and dont recomend driving around to far with it removed. all your trying to do is get the piston inside moving around and freeing it up because the same trash you get inside the master cylender is in the rabs, lines, and the calipers too. put the spring back in and see if it makes a difference for you because it cured everything up on mine. some people remove the spring and put a different one in but i dont because it wasnt designed to have a different spring in it, or you can buy a new one but they are quite pricey.