I'm wondering if the brake handle took such a hard hit, that it may have blown a seal in the master cylinder. Just a thought...

 

Sounds like an internal leak in the master cylinder to me. It used to happen on cars all the time. The seals in the rear master cylinder may be bad causing your problem and this is all internal so there is no outside leak or fluid loss. Could possibly be from the rear brake getting applied to hard as the other poster suggested.