Steam boilers in residences would normally be running at under 1lb pressure- if somebody cranked it up, that's a mistake. But it's probably not the root of the problem. It would be surprising to see a steam boilers pressure relief valve spilling water unless there was something really wrong (or grossly maladusted) with the operating pressure control. Steam PRVs on residential systems would normally be 8-15psi, (15psi being more typical) which is several times the ~1psi peak pressure you should see on the system with a correctly adjusted & functioning pressuretrol.
Hairline cracks in cast iron will leak at different rates depending on the temperature, and differences in temperarture across the boiler would cause it to open up further until the temperatures even-up. When you fill a warm boiler up with cool water the average temp of the boiler drops well below it's operating temp. While filling it the temperature differential is large, which may be why it leaks initially then tapers off when filling. When at full operating temp it may be opening up again. I don't know of a safe & sane temporary repair option here.
Any DIYer adjusting or repairing their steam heating system needs to own (and read) this book. It'll save you more aggravation & service calls than you might believe.