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.