High Pressure Valve?
 

Okay, really easy to find the low pressure side valve.

So, where is the high side valve located on an 89 GT?

Thanks!

Wow, crickets again...

Thanks for the help..........

I am not sure anyone has 100% certainty about what "valves" you are referring to.

There is a high and low pressure for your AC
There is a high and low pressure for your Fuel lines
Maybe your running boost and are referring to the air intake

I know from experience, the more detail you can put in yrou questions, the better chance you'll get the info you're looking for.