moving air
Front and rear air are really just 2 ends of one hose, so to speak.
your marginal performance up front will magnify losses to the rear. R134 systems really have to be "up" to work well. R12 could tranfer heat faster so a marginal charge would still cool. It is also possible for a R134 system to be ok and not work in the driveway, you need air moving over the condenser out front to get the thermodynamic cycle working. Check the condenser for any folding of fins and make sure the space between the radiator and condenser is clean.
Sidehackbob
78 93" flh
98 80" flt
93 K2500 6.5 TD auto K&N, straight pipe, 276K
78 K25 292+.080 ported headers 4bbl mucho rusto
79 F150 2wd 460 balanced ported, performer, 780 Vac, headers, duals, jacobs, mucho rusto 225K