spongy brakes

I'm having an issue with my brakes on my 66 gt jr. it is a simple, but modified, system, with a single master cylinder, no booster and alfaholics 4 pot big brakes in the front with stock brakes in the rear. The problem is, the brakes get spongy, then I bleed them and they are fine, then over the next 6 months, they get spongy again. I am not leaking any brake fluid. Where could air be getting in?

thanks