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You may find that you need to really purge each line of pipe individually, and you may not have the valves to do that easily (if at all). I can't really make out… but it may be possible with the balancing valves if they can close enough. And purging involves a full flow of water under pressure; the pressure reducing/maintaining valve may not be able to do that — and going out a drain somewhere. If you don't get wet, you probably aren't purging enough.

If the Schrader valve on the expansion tank is hissing, it may be possible to tighten it up and make it stop. It may not, too, in which case the tank may need replacing. In any case, the tank must be pressurized to the system cold pressure, either with the system drained or with the tank disconnected from the system and empty of water.

If the prv has been sitting unused for years it very likely is stuck especially since i think that model is steel or cast iron.

That setup on the tank is to make draining the tank a little easier. You do not need to do my first step from my previous post to drain the expansion tank. Someone else has made it easier. To start with an empty tank all you need to do is place the end of that hose in the floor drain or laundry tub.

1. Make sure valve #2 is closed
2. Open valve #4 and let the water out. Once the water has stopped flowing the pressure gauge on the boiler will be at or close to zero PSI
3. Open valve #1 and let air into the tank. You will hear air gurgling into the tank.
4. After some time the air starts to gurgle into the system, that means the tank is empty and you close all the valves.
5. Now open valve #2 to to add water to fill the system back to the proper pressure.

The two red valves are a relief valve and a pressure reducing valve the pressure reducing valve is also known as the automatic fill valve. It takes the house pressure of 30 to 70 PSI and reduces it to about 12 PSI. if it is working properly some folks leave the manual feed valve open so there is always 12 PSI pressure on the system. That way, if you are not getting any heat from any given radiator, you can just open the radiator vent and let air out. VIOLA »»» HEAT!

12psig is the amount of pressure needed to lift water to the top of a 2 story house more or less.

Looks like that version pf the prv doesn't have the fast fill lever. it could be stuck.

Is there a manual fill valve? can you show a better picture? I don't see anything bypassing the prv in your picture.

yeah, that is very nice work. Making it easier for the next guy is the finest way to show your craftsmanship.