Make The Right Choice
Brother of The Benz, The Bob
It's good to know where the schraeder valve fittings are, both high and low side of the system.
But, to add refrigerant to the system and to know what the evaporator pressure is, you better be concerned about the low side fitting.
Normally yoiu won't have concern about the high side as long as you don't over charge the system.
Good luck for even though R-134A doesn't cool as well as R-12, it does a good job.
Happy Trails Beep Beep from The Spiderman in Houston!!!