Time to rebuild/replace the master cylinder. If the seals or diaphram are old and hard, they're an accident waiting to happen. Pull the M/C apart and check for pitting/scoring on the piston bore; brake fluid traps moisture and may be eating away the metal from the inside (especially if it's been sitting a while).
Recheck each wheel cylinder, they could be suffering from the same internal corrosion and spitting tiny amounts of fluid; and it's a good idea to completely flush the hardline and replace everything that looks "iffy."
edit: Josiah, I read the post to mean the brake went to the floor before the new exhaust was installed. :confused:
