The rear seal on the master is leaking by. This usually does not effect the braking, but you will notice that fluid needs topping , but a leak can not be found. The fluid goes into the booster
and is aided by the vac of the booster .
Once the master is fixed, it is wise to put a small hose into the booster through the master mounting hole a suck the fluid out of the booster housing.
I have seen them fill right up... a sign is brake fluid in the booster vac hose.