Brakes losing pressure at red lights, any ideas?

You still have a leak!

You should visually check the front calipers. the rear wheel cylinders, and every inch of the brake lines for seepage. But I would bet on the master cylinder being the culprit. A "leaking" MC cover has no effect on brake performance because the piston should remain covered by brake fluid at all times. (unless the fluid is low)

This could get scary if you keep driving on that single chamber MC ! If it fails, you are SOL. I've driven cars when one circuit of a dual system fails and it's bad. If your single circuit suddenly has a complete failure, it may be "Hello Progressive" phone call time.

If it were me, I'd convert to a 73-75 A-body power brake MC but run manually. The 7/8" bore gives a very nice feel w/o the power brakes and is far safer.

I'm all for correctly restored cars but safety should come first. I'm converting my "new" survivor 64 Valiant to a dual system before it's even drivable.