A/C Diagnosis Help

I've gotten partway through diagnosing my A/C not working and before going down the rabbit hole further, I'd love a little advice on the next steps I'm taking. Before I loosen the airbox, there might be other things I can check.

Background
1979 308 GTS I've owned for 28 years
Used R12 until 2020 and was converted to R134a by my mechanic with 1.6 lbs of R134a
The conversion was just a new dryer and adding adapter ports to the compressor. Did not change compressor or fittings as far as I can tell

It has, at times, blown cold air so I know the system can work. But it doesn't work for long, which screams a leak.
In September 2021, my mechanic recharged the system for low R134a and did a leak check but didn't find anything. Evacuated system and added 1.76 lbs.
In July 2024, my mechanic saw low pressure, recovered the R134a and it was almost out but passed vacuum. He added dye, evacuated the system, and added 1.78 lbs.

So that brings me to today. I figure if the system is this finicky, I should just learn to fix the damn thing myself so I got a setup from HF of gauges, dye, vacuum pump, blacklight flashlight, and R134a.

Then the fun began as the true consequences of the R12->R134a port adapters became clear. I couldn't access the low-side port from the top and I can't access the high side port at all. But here we go down the rabbit hole

Car is currently on my lift with the wheel and wheelwell cover off.

First check - any charge in the system at all
I could get the low side port attached by dropping it from the top of the engine and attaching it from underneath.
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Car is off, ambient temperature is 93 degrees. Connected it with all of the valves off and then opened the valve at the compressor and got 104psi. Since the car isn't running, that's total system pressure not low side pressure. I think that tells me there is a reasonable charge in the system.

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So next step - is the clutch working

With the car off, I could rotate the clutch by hand.
Then I put a thermometer in the vents. It showed 93.7F.
Then I started the car, put the A/C coldness setting to the middle, turned the fans on high

Checked under the car and I could see the clutch plate turning with the belt. So the clutch seems to be engaged.

Then checked the radiator fan - it was running

Outlet temp after a few minutes , 90 degrees
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Ok, what about low side pressure

While running low side pressure started at 30.5 psi and dropped to 28 after a few minutes. That's definitely low. This chart says I should be seeing around 50 psi.


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I tried to check the high side pressure. Problem is that the high side port interferes with the airbox inlet.

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I thought removing the air intake rubber hose would do it, but still interfered. I can't yank the airbox to the side enough for the coupler to clear the corner of the airbox. So removed the airbox cover and realized I need to loosen those 16 nuts holding on the carb trumpets, and realized I could loosen 12 of them but don't have a tool to loosen the back 4. So I'm ordering one of the 8mm gearwrench ratcheting flex-head combo wrenches. (I have a ratcheting wrench, but it doesn't have a swivel)

So a few points of advice would be helpful

1) Would a low charge alone (28psi vs 50 psi) be enough to have the system essentially not functional and give only a 3 degree drop from ambient (93 ambient -> 90 degrees vent temp)

2) System obviously has a leak so tonight when it's dark I'm going to take a look around for some dye around the dryer area, the compressor fittings and clutch, and the condenser in the grille. Anywhere else I should check?

3) If I can't find any dye indicating the leak, my sense is to get a high side reading which means dealing with this damn interference. Is that the right thing to do next? My plan is to loosen the carb trumpet nuts and hope the airbox will then be able to be shifted. For those who have done this, will it shift or do I need to actually remove it? I've never removed the airbox myself and would prefer not to risk dropping washers down the carbs. It looks like that port could potentially be swiveled but its a tight enough space that it might require removing the airbox to do that and I'm not sure about loosening the fitting while the system is pressurized.

4) Anything else to check before getting a high side reading?



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