If the fluid is not leaking anywhere then you could have a bad master
cylinder that is leaking internally.
Other Problems that could cause this:
1. Brake shoe in rear brake assembly has come out of place and is allowing
the wheel cylinder to keep expanding (just happened to a customer last week).
2. Brake hoses are bad and are ballooning.
I have all of these parts (with pictures for easy identification) on my
website (see url below).

Hope this helps,
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242