My bad, you will have clearance issues with a gun. Breaker bar and a sledge hammer is what you need. Put a dab of RTV on the threads when you put it back in. Trying to do your front seal?
Not joking, I do have a lot of experience, mostly being a smart ass though.
Seriously, F-1,GTP builder, A+P mechanic, electrical contractor, open wheel driver, track records everywhere I went. Never had the need for a torque wrench, but my hands are so good, sorry. Wait, I left out like fifty...
Mechanics do not use torque wrenches, but you better get one. I told you 1/2'' impact gun, $29 harbor freight and all the air pressure you got. Better yet, STOP WHAT YOU'RE DOING, LEAVE IT ALONE.
Hitting different areas with break parts cleaner when the engine is running, alt bearing, idler pulley, compressor, good way to diagnose. Check rotors for non matching temps, love them temp guns.
Reading your post and what you own, I think you know what you're doing. I say the pump is NFG, or a line is blocked?
Check valve? air in cooler? fluid should be swirling in reservoir at idle. I would start cracking open some fittings and looking for pressure. That pump should whine and be a...
You need to put air pressure in that cylinder and see or hear where it is going. Guessing the head will need to come off, cheap pair of valves and head gasket and maybe good to go, 100psi is not enough.