I've also seen master cylinders leak down the firewall, check for oilyness and/or peeling paint below the MC. Could be a leaking wheel cylinder or brake hose, as already mentioned.
'68 GTO, 400/TH400, AC/PS/PB/PW, hideaways, Hurst dual gate
'72 442 'Vert, 350/TH350, AC/PS/PB/PW
'74 Corvette, 383/4-speed, 450 hp, restomod
'71 Javelin, 304/3-speed
'70 Cougar XR7 'Vert, 351W/FMX auto *SOLD*