Brakes fall to the floor!!!!

[hugger6933]

I am finishing up a 03rcswb 2wd c1500[for those of you who don't know me I build a lot of late models, but hang out in the 67-72 forum]. The truck is lowered 4/6 and has 20 inch wheels has a 4.8 auto and is about ready for paint and a new home. Sometimes when you mash the brake pedal it will fall to the floor. no brakes! when that happens you can pump then up and they will work fine for some time[maybe days] before it does it agian.I changed the mastewr cylinder because the booster was a little wet at the mateing suface for the m/c booster. I thought it was fixed drove it a couple of days with no problems then it happened again. Any thoughts? I thinking the abs pump under the cab? Just not sure . Please help. Jim