why is there no pressure when engine is hot and overheating

You seem to have done all the right things. But did you get a new radiator cap? If the spring is weak it will allow the pressure out

I am having the same problem. I have replaced reservoir cap a couple times and believe this one can hold the pressure. The auto mechanic says that i have got to bleed out of the system all of the air pockets. I think i have most of it but the reservoir height when at full is lower than the bleeder screw on the coolant output port on the top of the engine. I am now looking for a pump to pressurize the water at the reservoir to be able to bleed all of the air out of the system. Ugh!

I am able to prevent my cooling system from boiling by turning the heater and fan on full inside the car, I have made certain that the outside air goes through my radiator by buttoning up the nose of the car. When I place my hand behind the radiator/fan the air coming through the radiator is cold.. I have replaced the thermostat twice as it controls the hot water flow through the radiator but not the driving compartment heater flow. Still the mechanic says that it the air pockets causing the thermostat to not open properly. Im gonna find a way to pressurize the system as I said above. If that doesn't work I might drill a 3/16th hole in my thermostat. I am getting that desperate. Don't judge me.

So I went to Autozone and rented the pressurizer pump and was able to bleed some air from the system but now I am thinking this is all for not because when I take the pump off I am able to squeeze the hose pushing water into my reservoir and when I let go the air goes back into that upper hose. This because a squeeze on that hose puts water into the top of the reservoir where there is air since it is above the full line. So I will try a 3/16 hole in the thermostat. Any more reasonable suggestions?