[T2] Speaking of leaks- brake fluid "disappearing"

[MUTS]

I surmise a crack in/below brake reservoir, brake fluid is going down 
and I now keep filling it, but nothing is leaking on the ground or where 
plastic tank sits after I added sculpy clay all around the bottom of it. 
Power brake assist is working ok- once I did have very spongy braking 
(after sitting for long time?), filled it up, pumped it a lot, bubbles 
came up, fluid went down and pedal was hard.  Where is the brake fluid 
going?  Somewhere in body below the plastic reservoir?

MUTS  = Michael's Ultimate Thumb Saver oboe, clarinet support
93 Chevy Roadtrek 210P
78 VW Westfalia Camper