Hi Dan, Your situation is very common, ie previous mechanic trying to work on a Rolls and has no clue. You are on the right path coming here for accurate advicea and direction. The G valve controls rear wheel lock-up during sudden braking and is known to hang up with debris because it is in the rear brake circuit. The height control system is part of circuit #2 and is mostly passive unless passengers or a load is in the trunk and triggered into fast mode via the height control solenoid valve letting in hi pressure to compensate for the weight.
Completely separate from both brake circuit but shares the pressure and rear reservoir.
1980 SWII LRL-41587C FInj
1976 SS LWB LRE-23114 Harkness RR