It may not be too late to apply the grease even if it starts hissing. If the seal is not pulled out or cracked the grease will slightly expand the rubber and stop it leaking.
And yes it is possible to replace the seal without removing the servo!
I've done it on another car, you need to:
-disconnect the pin connecting the brake pedal
-pull the grey gaiter off
-pull out the old seal with claw like tool (or a bent wire etc)
-lubricate new seal and gradually push it in with something smooth without any rough edges. Other end of a thick makeup brush did it for me.
-put the gaiter back on
-put the brake pedal pin back through-i strongly advise to replace the clip for a new one-you don't want to lose your brakes if the old bent clip brakes!
There's very limited space and you'll end up scratching your hands laying on the floor with a cramp in your leg and you won't be able to move lol.
But it's much faster and simpler than dropping the engine
