Mark IV Roamer AC refrigerant amount

[Clownpuncher2]

Hey there long time lurker first time poster.
I have a 70 gmc c1500 with a Mark IV Roamer AC unit that I'm in the middle of restoring. Does anyone know how much R-12 refrigerant is required to refill the whole ac system? I am converting it to R134A so I will have to fill it to 85%.

Thanks.
Dwayne

[Myself]

I always just watch my gauges for anything goofy and try to get my low side at 35-40 psi while at around 1500rpm. If I get good cold flash and sweat at 35psi I stop there and they work great. Can upside down, shut off the valve on your manifold every now and then to watch a true reading in the system.

[Wrenchbender Ret]

Most of these old units have a sight glass. It is usually on top of the drier. As you are filling the glass will have a foamy look. When it clears up it will be full. After it is full put in just a little more. They usually hold about 2 cans of r-12.
George

[Myself]

But.....if going to 134a you can't simply watch the sight glass until it clears up. I've noticed with proper pressures and things oerating properly that there will still be bubbles in the glass with the 134a.

[engineer_gregh]

I also recommend flushing the system before refilling. R134 and R12 are not compatible and use different oil. You'll need PAG 150 oil for R134. Any of the big box parts stores loan the tool needed for flushing, they also loan gauges and vacuum pumps. As "myself" recommended you want your low side pressure at 35-40 if the outside ambient temperate is 70 degrees. I pasted the link for a good chart below. I also run a box fan or similar in front of my grill to help keep engine overheating down on my big block trucks (they run warmer than I like if idling with the AC on for extended periods). Hope this helps.

http://acprocold.com/faq/r-134a-system-pressure-chart/