boiler ignites but water doesn't circulate

I have been trying unsucessfully to eliminate: low noise level banging, gurgling, and uneven heat temperature at the two pipes that Y into one before the circulator and a noisey expansion tank for many years. My system is in the neighborhood of 40-50 years old. Home is a one and a half story cap cod with the original hot water baseboard system expanded into the attic ( 2 bedrooms / hallway between) and basement (family room, bedroom,utility room, bathroom) some 20 years back as part of a dyi as the family grew. I keep purging system runs fine and quite then maybe hours later or the next day or so, the sounds return. I have a time & temperature set back thermistate and it is most common to hear this noise upon heat startup at 5:30am after sitting idle from 1:00am. I have to tune the cut offs before they join the single pipe to the circulator to get consistant heat temperature. If I have both valves full open then always one pipe will remain just warm only until the circulator is just about ending, while the other is HOT throughout the cycle. If I adjust the turn valve (on the one that gets hot) to say about 1/4 - 1/3 open then both pipes are equally HOT throughout all phases. AIR? My other question and concern is the expansion tank. I replaced the original maybe 15 + plus years back (on original dain valve was leaking and could not remove) it was earier to just get another and sweat to the exisisting 1/2" copper line. When the system kicks on and the boiler starts to heat the line going to the tank becomes extremely hot (while the boiler is heating) only. The path from the top of the boiler is the copper line tees at a fitting to the pressure relief valve, travels up, right angles over, angles up again, have inline supply turn off valve 8" before tank entrance. Tank is round 12" dia. and 36"lg. and has a drain valve on one end. This tank can also be heard and felt popping and bang and as I said earlier the pipe leading to it are hotter then heck. Once the boiler kicks off everything is normal and the pipe quickly cools off. Ive always felt that if the system was getting to much pressure the relief valve would blow and I have check this function on more than one occasion. Is this style of tank suppose to have water in it. If so how much and how would you control the amount since you can't see inside. If it is not suppose to have water than should I close the valve that is 8" below the inlet and open drain valve on the 12" diameter end. This would remove most but not all of the water since the drain valve is about 1" up from the tank bottom side edge and the hose bib inlet is pointing towards the ceiling plus the supply valve would have the 8" of water. Any help to clarifiy issues would be really great.

Does anyone here know if there is a flush available for a hydronic(hot water) heating system? I find that when I bleed my rads I get some copper/black colored water which makes quite a mess. If the chemical is available what is the proper flushing procedure? Have a wonderful Christmas and a terrific New Year. Vic