I have a 1998 s10 2.2 that doesn't have heat. I've changed thermostat from a previous stuck open thermostat and still no heat. While trying to figure this out I noticed that the radiator doesn't build pressure. I can drive for a hour and can take radiator cap off without any coolant rushing out. I've also let it idle for 30+ minutes without it boiling over without the cap. Water pump is new as well. Any suggestions?