90. servo vacuum

[Wagman]

Have recently bought a 1988, 90. It's all the usual,but with one inconvenience I can't get to the bottom of. I live in Sapin and am having difficulty finding the right help in my area.

Every couple of days, usually when the steering is working hard (tight reversing with a loaded trailer, into my garage for example) there seems to be a build up in pressure resulting in the loss of all the fluid from the servo resevoir. Usually squeezed from the top of a jubilee clipped hose.
Immediatly refilled theres no more problems for a while.

I have thought of just putting a small hole in the filler cap but don't know if that is the way to go. Is there a bigger problem I should be addressing?

I'm no mechanic, and this is my first Landrover, but, with a workshop manual and the help of the forum I'm brave (or stupid) enough to tackle just about anything. Any ideas!!