If your parking brake starts sporadically engaging or won't disengage at all, then your engine speed sensor could be bad, disconnected or making a poor connection. This took me about 45 minutes of research, and about 5 minutes to fix since my sensor had just become disconnected.
The machine needs about 400 RPMs at a minimum to release the parking brake, so if your sensor is not sending an RPM signal, the brakes will not release.
