I have a 66 chevelle with perfect fit ac system.
I'm charging the system for the first time.
The directions with the sytstem say charge with 24 oz of 134a.
I did that and only gettting 11-12 lbs PSI on low side and not cooling, so I added 12 more oz and it brought it up to 14-15 psi on the low side.
Does anyone know what the pressures I should be shooting for?
Thanks in advance

Did you have the engine speed up to 1500 rpm and a fan in front of the rad to get a good reading. Use a thermometer in the right ac vent for the temp.
Should get 40*-50* air from the vent and low side 26-29 with this setup. Do not put too much 134a in as it will not cool, more is not better, actually a little less is better.