cooling system ?

[Andy XR8]

changed coolant on the weekend and now the system won't pressurise. i've tryed leaving the cap off, undoing what i think is a bleeder valve, replacing the cap and every time i take it for a drive to heat it up, (it never over heats, doesn't even go near half way just stays in the same place it always did), i can take the cap off like it was cold. please help.
The system is supposed to have pressure in it right?