Brakes losing pressure at red lights, any ideas?

I have a 66 Barracuda with Kelsey Hayes front discs and original rear drums. It still has the original single reservoir master cylinder. I recently rebuilt the proportioning valve that was leaking. The proportioning valve is no longer leaking and the car stops well, but now the brakes are losing pressure at stop lights. After a minute of two of keeping the pedal depressed, it starts losing pressure. One pump and it's back up.

I bled all four corners and didn't get any air and it isn't losing any fluid that I can see.

Could air be leaking out of the master cylinder cap? It looks completely original. What's the best way to make a new gasket for inside the cap?

Thanks for your help!!