mine makes a quick psst when I open it sometimes.
danger?

 

The cooling system will build pressure as you heat the coolant due to liquid expansion. When it is cools and contracts, there should be very little residual pressure.

If you are worried, tee in a hose and a pressure gauge so you can get actual pressure readings (numbers) when both cold and hot.

 

Also, after the truck is warmed fully, release the pressure and run it again. Cold air and water will expand with heat and give an erroneous pressure reading. Mine will read 10-11psi after warming from cold then I release the pressure in the system and run again, resulting in a max pressure of 4 psi.