Consider Jeep went to the “modern” disc/drum arrangement back in 1977 on CJs and in 1974 on FSJs. That means even the newest drum/drum system is 34 years old. As the vehicle sits, moisture permeates the fluid and rusts the master and brake cylinders internally. The brake cylinder seals can dry out, springs can rust and snap, and any number of other age-related issues can occur. Start your inspection by popping the master cylinder open and inspecting the fluid volume and condition.