If steam shot out when you popped the cap, you have air pockets. This is probably the demon you've been trying to exercise. I had a customer with an Escort a while back with a similar problem. I told him he had air pockets; he wouldn't believe me, then he came in a short time later and found that was the problem after all.
I'd go ahead and do what you suggested. That being you let it run with the cap off, and a fresh mix of 50/50 ready to fill it. I'm willing to bet the water level will go down quite a bit. You might wanna try doing this on even ground, too. Good luck.
p.s. Is your pump the proper rotation???
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch 