2000 blazer 4x4x4. Does anyone know if you can change the high pressure port valve or do you have to change the whole tube? I have a leak that is coming from the little black ball in the valve.

 

Yes, you can change the schrader valve inside the port on both the high and the low side. Looks like a large tire valve core. You'll need a special tool to change it without dumping the charge in the system. The tool is about $40 on up each.

It may just need to be tightened a little, you can get a tool for that for 5-10 bucks.