Adding water frequently to steam boiler

background:

Brick three story built in 1906 near Harrisburg PA. One pipe steam, Burnham Independence IN-4 steam boiler. I’m running a vapostsat set at 0.5 psi. Both main outlets have risers piped to a drop header.

Return piping, some was in bad condition, I removed it and replaced with new.

I keep an eye on the water level and add water, about a quart every week more or less.

I have examined the radiators, the valves, the radiator vents, returns, main vents, supply lines, and the boiler, both by overfilling it and by removing the top housing and examining the castings for leaks. The firebox has no water in it either.

Other than the leaky return piping that was replaced, I have not found any leaks.

The system works pretty good, with steam getting to the radiators and providing USUALLY fairly even heating to the 10 radiators. One of my living room radiators has many sections and is about four feet long, and seldom do any of the second 1/2 of the sections become warm! No banging, hissing, just normal expansion/contraction sounds including clicking/tapping sounds.

I used a mirror over the top of the three main vents that I installed in a manifold I created at the end of my single main and none appeared to be Leaking water or passing steam.

Knowing it’s bad to add water this frequently, I would like to eliminate the leak(s) but I cannot locate any, I’m stumped, please help.