98 deville block test??

I bought a 98 deville with a 4.6 that is overheating, and am trying to do a block test to check head gaskets/bolts. The instructions on the tester said to have coolant below 2" the cap, so I siphoned all the coolant out of the tank, ran engine at idle at 195 degrees(when I started). The fluid stayed blue for about a minute and a half, then I started sucking coolant into the tester. the tank had filled back up. anybody know what I'm doing wrong? The guy I bought it from SAID he had replaced the radiator, water pump and thermostat. btw this has green antifreeze in it. any help is appreciated thanks

What tester are you using? No info here!! If it is the type that uses the special fluid that turns yellow to signal exhaust gasses that is the most common.

What tester are you using? No info here!! If it is the type that uses the special fluid that turns yellow to signal exhaust gasses that is the most common.
sorry, that's the one I was trying to use, I've watched the video on you tube several times , but it doesn't address the tank filling back up

What is the issue wiyh the tank filling up if the fluid does not change to color when drawn into the test tube then there is no issue. Another test is start the engine cold, the rad hoses would be soft, if after say 3 minutes they are hard, then the system is being pressurized by a bad head gasket.

the hollow bolt was plugged