From the size of the connector on that hose, it looks like its designed for the 134a fittings that are installed on all vehicles manufactured from 1995 on. (A handful of vehicles used them about a year early.) Your bronco will have a standard 1/4" flare fitting on the suction side, usually on the suction line near the accumulator. The reason to convert the system is that 134a requires POE oil rather than the mineral oil that you have in the system. The molecules are also waaay smaller requiring a different seal compound to keep the stuff from leaking out real quick. Just shooting134a into the system may make it work a little better but you should at least remove all of the existing refrigerant from the system and evacuate it. There are several drop-in replacements for R-12(which is what you have now) but they're not approved for use in vehicles by ASE because they contain traces of butane. You would still be better off using one of the drop-in replacements for R-12.(Hopefully none of the ill informed know-it-alls won't read this and start the pure butane refrigerant rolling bomb debate again.)
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