Lot's of causes for a low oil pressure warning light. Sensor issue, plugged pickup tube, collapsed oil filter, sludge in oil galleys or actual oil pump failure.
You need to find the oil pressure specs for specific engine rpm, there's usually a plug that's removed on the engine to tap into to get oil pressure readings on a gauge. If it's low you have a mechanical issue. No change then you'll have to look at electrical.
Power brakes are powered by a vacuum pump, look at that system, they may not actually be related although oil pressure failure may cause the pump to seize, I don't remember if the pump is part of the engine or separately driven.