I got a new cap with a new seal and put it on yesterday. I didn't have enough R134a to completely refill the system, but I got enough in to blow cool air. I figure if it stays under pressure for a few days, I should be good to go.

I also found a site online selling 12 oz cans for $3.99 each. This sure beats Walmart's $10.97 price.

I expect to be able to fully charge the system in a few days.