the solenoid valve at the same time is a one way valve. dahil hindi makakabalik ang refrigerant on its reverse direction, the rear evaporator gets more than enough cooler refrigerant that passes by the rear condenser. malamig pa rin naman both front and rear kahit walang solenoid valve. pero masmalamig kung meron nito. probably in addition to your problem, the rear expansion valve is defective or the nissan evaporator coil is not matched to cool down the rear cabin. maybe konti lang ang rows ng coil which in effect gives lower level of coolness. how about the rear condenser, nilinis din ba?