After answering a blog comment, it occurred to me that my experiences might save someone else some grief, so here is the answer I gave:
DIY has saved a lot of money, but I feel my ignorance is causing some of my pain. Thank God for the friendly neighborhood plumbing supplier. I now know so much more about how the heating system works. Some of my assumptions were so far off base, it's amazing I didn't wreck the boiler.
I thought the circulating pump provided the pressure. Wrong, the pressure reduction valve does that. The pump simply makes the water move around, not up. It's the pressure in the boiler that fills the system.
Pressure in the boiler can safely go to 20 psi, with the relief valve letting go at 30 psi. Mine let go at 8. :/
It takes less than five minutes to fill the system with the valve wide open, not two days. :O
You bleed baseboard radiators by purging them with a garden hose. Do NOT try it without the hose. Hot water sprays out like a fire hose, and when the air pocket hits, the water mists. Trust me on this.
And the iron barrel in the floor joists above the boiler is supposed to have some water in it. :}
Thanks for reading.