Sounds like a bad master cyl. or you could still have alot of air in the system.

Try gravity bleeding the whol system. crack all four bleeder screws and let gravity do the work. make sure you keep an eye on the fluid level.

The brake light comes on when the fluid level is low in the master cyl.

If your master cyl was weak, bleeding the brakes may have wiped it out. I would pull the master cyl and check if it is leaking into the booster.

Good luck