what brake fluid are you using? also your rear rotors might be glazed over, if they are u need to have them turned or replaced,  do you have a temp gun to see how hot the rotors are? also this could happen if the brake fluid might be close to the headers and the heat can break down the viscosity of the fluid causing failure AND if your using plastic brake lines, use standard steel lines if possible.  Hope this helps!

One other thing.  If you are running a floater read end.....  Make sure your hubs are tight.  If they are loose the rotor will flip back and forth and push the piston back into the caliper then it will take an extra pump of the brakes to get it back to the rotor.  This is a very common issue.