The Hunter's hyd pump wasn't capable of maintaining pressure to the hydroboosters if other services were operated at the same time at low throttle angles. So idle thrust, airbrake in and landing gear down at the same time when rolling out downwind off the break would invariably cause manual reversion until the airbrake and landing gear had completed their operation.
VERY unusual for a 'momentary drop in hydraulic pressure' for any other reason, apart from inadequate maintenance, possibly?