The system is probably very old.
What should be done is, in theory, provided no oil is let out of the system and any part that needs to be removed, only reinsert the quantity of oil that came out along with the refrigerant during the refrigerant evacuation process (plus, of course, any other oil quantity that has been removed from any component that needed to be replaced). Obviously with the correct oil type and viscosity (i.e. if the compressor needs PAG 46 oil, PAG 150, PAO 68 or POE 55 oils won't do any good to it).
However, provided there's enough time at hand, the best option would be to take advantage of the situation to flush the whole system in order to remove all of the old oil, replace the dryer/accumulator, and then insert the required quantity and type of fresh oil and follow the required oil balancing procedure (which usually also includes rotating the compressor clutch hub by hand a certain number of times so that it won't be pumping any liquid oil while starting up) so that the compressor would be lubricated properly and won't seize up during startup.
Another important thing: if the rear evaporator hasn't got a solenoid valve that cuts the refrigerant when it's not in use, since you'll probably get access to it during your repairs, make sure that the TXV thermostatic bulb is making PERFECT contact with the evaporator's outlet; if it's not making perfect contact, you risk slugging the compressor with liquid refrigerant when the front AC system is on and blasting cold air, but the rear AC blower is off, because the rear evaporator's TXV's needle won't be closing up when the thermal load abruptly decreases. Very important with parallel evaporator systems (in cars, systems featuring rear ac basically).
Above all, only trained and licensed professionals should work on AC systems. That way, there's no risk of doing any damage to the system, or letting refrigerant vent into the atmosphere.