Manifold gauge high side question

1. After connecting the high and low side with both manifold valves close, I start the compressor to get my reading, if low side is 28 or below I connect a can of 134 on the center yellow hose I then and only then open the low side valve to fill system till a reading around 45psi at 95 degrees at suburban grill. My question is do I ever open the high side valve, if not whats the high side valve for?
2. My next question is I will be using r-12 guages with 134 adapters jumper hoses, will it still read right?
3. My problem is I have a 1999 suburban that has slightly cool air in rear control vents but only warm air in front control vents. could it be a stuck damper door in front or just low freon?

 

high side valve is only for pulling a system vac. never open it with the system filled!!!

could be doors, could be a bad orifice tube, pressure when running on the low side should be no higher than about 45 psi. with the compressor engaged.

murphy was an optimist

 

I had the orifice tube replace last month and the old one didn't look that bad. At the pipe where the orifice tube is, the pipe is very cold but only cool air at the rear vents and only warm at the front vents. Could the front dash control switch be bad, is there a thermstat at this dash control switch?

 

could be a blend door issue ......but its hard to say without knowing both high and low side pressure as well as clutch cycling rate .

if all pressures are in spec and your cycle rate (clutch on time , then clutch off time )are in spec then you need to check your temp or blend door operation .

the high side port is used primarily to help diagnose the refrigerant system IE: an abnormally high or low pressure would give us an indication as what might be wrong with the system .

never open the high side port valve with the compressor running as the pressure on the high side could be sufficient to cause your refrigerant can to explode .

45 psi on the low side is on the upper edge of what we like to see , usually we like to see less than 30 psi , most compressors will shut off at around 22 psi or so to prevent the evaporator from freezing up ..generally speaking the lower the low side pressure (while maintaining proper cycle rate )the lower the output temp .