Bought a L.C. master cylinder from Marlin about a month ago..this is going on a 81 mini w/ L.c> rotor and ifs cal.. new wheel cylinders in rear. here is the problem. ican't get the pressure to hold. i bleed everything and on the pedal it feels like it builds up and then leaks out but i have no leaks. i'm at a lost . thanks ahead for any help.