disc brake pedal low

i have just changed my 1970 duster over to front disc brakes i installed a set of rebult kelsey hayes 4 piston caliers installed new disc brake proportioning valve. on the first test drive the brake pedal went half way to the floor but when i pumped the pedal they were to the top with good braking the problem is when the car set for some time the first pump on the pedel is again half way to the floor i have bleeded the lines but has not helped i dont know if i still have air in the lines or wright master clyinder the one i have on the car wright now is a tandem with one large reservoir for the front brakes and one small one for the back