A/c works sometimes

Hey guys,

I am new here!

So my parents bought this 2008 Prius a few years ago. When they first bought it (used), everything seemed to work just fine. Now I have the car, and the I noticed the vents were blowing out hot air when the A/c was supposed to be on (every time, never cold air). I was recommended to try A/C Pro refrigerant to try and repair it. I tried but when I attached it to the low side pressure port, it was giving me some pretty strange readings. Recommended 25-50, reading 50 when the car was off, and around 70-80 when on, and more if I turned on the A/C. We tried opening up the valve to the A/C Pro since we figured if the PSI of the car was higher, thus the pressures would equal out instead of putting more into the car. This seemed to work a little, now they work sometimes usually not when it's really hot outside. It also doesn't really matter if I'm just driving on the streets, or on the freeway or if I have been driving for a little (because I know some older cars take a minute for the A/C kick in). I am not really sure what to do and would like to avoid taking it in for diagnostics if I can