Forced hot water - baseboard heat problem.

I have a small problem I need help with. I have forced hot water with baseboard heat. My expansion tank sprung a leak last week. I replaced it by spinning the old one off and spinning the new one on. In the process I lost some water. My problem is that now when the heat comes on I hear water running through the baseboards. I know there's a way to bleed the zones but I have forgoten how. I also remember it has something to do with the faucet and shut off valves at the end of each zones run back to the furnace. One last thing, on the top of the manifold that the expansion tank attaches to, there's a small round part with what looks like a tire valve stem on the top of it. It has a cap on the stem and when I loosen the cap, water starts dripping out. Is this some kind of bleeder ??? Do I leave the cap loose and let it drip ????
Thanks Brian
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The easy way, is to turn the electric off, relieve the system pressure, replace that "small round part with what looks like a tire valve stem on the top of it" (automatic air vent), add water to the system, turn the electric on, and wait a few days. The cap is just there to close it off if it starts leaking. It should normally be loose. Most times, the air will eventually work it's way around and come out that vent.
The harder, but quicker and more positive way of getting the air out, is to first turn off the electric, then close all (return line) shut off valves that are between the "faucet" (boiler drain valve) and the heater. Then put a hose on one of the drains, purge the air out by opening the drain valve, repeat the process on each zone, and then open the shut off valves. With this method, you need to maintain constant system pressure.

baseboard heat. My expansion

and spinning the new one

heat comes on I hear water

but I have forgoten how. I

at the end of each zones

the expansion tank

stem on the top of it. It

out. Is this some kind of

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