there may not be a high side fitting, many volvos don't have one or the other. They are supposed to be pressure charged with a high dollar machine at the dealership, but as long as you have a low fitting, you can charge it. If you don't have a real gauge, at least get one of those cheapie can tap low side gauges and you'll be able to get an accrate fill. Don't do it in th eheat of the day, evening out of direct sun, and if you have a box fan, point it toward the condensor
If you look on www.aircondition.com, you can find a temp presure chart for static charge and another that shows charge low side pressures at different ambient temps( those cheapie gauges sho over 45 being overcharged, but if it's really hot out, 45-55 is normal)

If the thing still has any presure ( check it late at night when cooler, while it's hot out, even air in th esystem will pressureize ) pop the schraeder valve and see if there's anythingin it ( don't breathe the stuff ) If refrigerant remains in the system, you can get away with charging, if not, you will need to vacuum it, and most likely ned to replace some seals. That sealer stuff only works on very slow seal leaks and it takes a long time for full effect ( I have seen it drastically reduce a compresor seal leak...over a year and a half ) if it's been empty for a while, the orings will all likely be shot
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-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '92 Ford F350 diesel dually