What I found works good is to remove the housing and thermostat. Then fill the hole with coolant until it'* right to the top. The put the thermostat in. That way you have no chance of a air pocket under that will prevent it from opening.

unfortunately, after filling it, all seemed well, (almost posted the yipee!), but now there'* leakage. is there a thread for water pump, etc?

Where is it leaking from?

it looks like it might be this https://www.gmforum.com/t284219/ I was going to fill it and see, it was dark when i noticed the leak, and it seems to be a very slow leak

would that also explain the lack of heat in the passenger compartment?

Lack of water explains lack of heat.

A leak could possibly introduce air into your coolant system, which might explain the lack of heat in the passenger compartment. Bit of a long shot though.

The heat exchanger for the passenger compartment is about the highest point in the cooling system. So any air can end up there. Ensure you have no air in the system. Fix the leaking elbow first. Chances are the O ring is leaking. Make sure you put a little lube on the O ring before you put it in.

thanks! what is a 'good lube' to use?

Lite oil, even a thin coat of Vaseline. It'* just so the rubber slides easy and the O ring wont roll or tear.