15 pound cap is stock pressure. I run one about half that much. A high pressure cap will let the coolant heat up to above 212F without boiling, but it also puts a lot of pressure on the whole system, rad, cap, hoses, head and crossover gaskets, pump and seals etc. My car runs at below 190F I don't need to put more pressure in the system.
Your cap may be bad but if it didn't let something out, fluid or air, once it got hot it could cause trouble. Maybe the system had some air pockets and the rad cap burped out the air, then when it cooled off drew the air back in. It's common to put some fluid into the tank when it is cold, 1/4-1/3 full, that way if some air was drained out to the tank it would be replaced with coolant. If you keep putting coolant into the tank and the level keeps dropping, you may have a leak elsewhere. Try another cap just in case that one's kaputski
That's a nice looking tank, could pass for a stock item. You still have your battery in the stock location? What's with the hole in the fender-rad support bar, have something bolted there?
