Vacuumed and recharged A/C with correct weight - pressures still low
 

I've rebuilt the A/C system on my F-250, so new compressor, orifice tube, condenser, dryer, valves, and flushed evap and lines. I pulled a vacuum for about an hour, let it sit for 10 min or so and verified no leaks, and recharged by weight specified on the sticker under the hood. The A/C is 'meh' and the pressures (both sides) were low compared to what I see on the charts. I don't exactly remember what they were, but the pressures were both in range for about 5 - 10 degrees cooler than it was outside based on the chart.

Similarly, I just vacuumed and recharged (no replacement components because there didn't seem to be any blockages) on my '08 Trailblazer just yesterday. Same thing. I vacuumed it out really good, let it sit for a while, and charged it up with the specified 33oz of refrigerant. It was 80f outside, so according to the charts, it should've been 40-50/175-210. According to my gauges, it was ~33/155 and the vent temp (at idle) was about 45f. When I drove around a little, I could maybe get it down to almost 40f, so that's 'ok', but not great. Since the same thing happened on both my F-250 and Trailblazer, I got to wondering if my gauges were bad, so I hooked up one of those cheap little low side dials you get on some of those regharge kits just to get a second opinion and it read about the same. What gives? Are these charts wrong? Should I keep shoving more refrigerant in there until I'm within the pressure ranges on these charts? I know pressures can also vary *slightly* with humidity (it's humid here), but only by a couple psi and not terribly significant.

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How much did you put in by weight?

For my F-250, I don't remember off the top of my head, but it was whatever is on the sticker under the hood. Somewhere around 36 or 38oz, I think...?

At what rpms did you read pressure?
the chart does not state rpms used but generally they are read at higher rpms.
40 degrees while driving is not bad especially if its very hot out. You have to be driving for a long while at high speeds to do better. Also check your airflow through the condenser to make sure it's as it should be. Dirt, leaves, and debris or a weak fan clutch will hurt your results.

Actually, local relative humidity should have no effect on either pressures or outlet air temp. Personal comfort, sure, but the physics in the refrigeration are unaffected by humidity unless it causes the evaporator to freeze up.

FSM says 42 oz R134a is the correct charge for a 99 F-SD. If you have a label that says differently, follow the label.

If your F250 doesn't have the vacuum operated valve in the heater hoses, the damned heater will re-warm the air making it appear the AC is performing poorly. I used a pair of vice-grips with some wood shims (to protect the hose from abrasion) to clamp off the flow of coolant into the heater core in the summer. Finally bought a pair of specialty hose clamp vice grips at Harbor Freight to do the same job without damaging the hoses. I put them on the heater hose in mid May and put them in the glovebox in September. Makes a huge (5-15 degree) in outlet air temp in the heat of summer.

Really need actual pressure numbers, outlet air temps, and ambient temps in order to analyze A/C system performance. You also MUST use whatever setup or perquisites that the FSM specifies in order to assure consistency. See attachment.

The multi sources I checked has the super duty at 42oz.