When you take the booster line off, you can make a dip stick out of a piece of plastic tubing and stick it in there. When it bottoms out, blow in the tubing and listen for fluid/air bubbling. That will tell you if any is in there .. then you use same tube to suck it out [ w/vac pump ]
I have seen them fill right up to the vac pipe hole...
The remedy is repacement of rear seal in Master ...

Careful not to puncture the diaphram....