Brake problem

Hello All,

Having problems with my brakes getting spongy a week or 2 after bleeding. I have a 74 Duster that was drum brakes all the way around. Converted front to disc with kit from Pirate-Jack (MBM Brakes), switched rear to large bolt axles and drum kit from Dr. Diff, mounted a new Raybestos master with 15/16 bore, and replaced all lines. Master bled and I bleed the brakes further-est wheel first with assistant pumping. Everything feels good for a couple weeks and then the pedal starts to feel spongy and takes 2 or more pumps to firm up. As always, I inspect all connections and can't find any signs of leaks. Re-bleed, get some air, and everything is fine for week or 2. Last time bled the brakes felt good for about 3 weeks and reverted back to the same symptoms. Am I missing something here or doing something wrong?