I have a similar problem, where the grommet on the servo has deteriorated and split. Unable to get a grommet, I am obtaining a second user servo unit and master cylinder. I was also considering buying a new vacuum pump, having read that it allows oil into the servo.
However, before I purchase a new vacuum pump, as mine still works, I have carried out some research, I have also taken apart my original vacuum pump (I replaced the original with a second user knee, about 18 months ago), the vacuum pump runs in oil mist from the engine, which usually returns to the engine. The research I have done, would suggest that the non-return valve attached to the servo, is more likely the problem. This valve seals off the engine from the servo, if this valve stops working, then it is likely to let oil into the servo, causing the problems which have been reported.
The non-return valve is installed in the hose between the brake servo and the intake manifold or the vacuum pump of the vehicle’s engine. When the engine is switched off, the valve closes to maintain vacuum in the brake servo, storing brake-assisting energy until the engine is restarted. The non-return valve also prevents fuel and oil fumes from entering the brake servo and damaging its internal rubber diaphragm.
