The design of the drive motor on the 863 & 873 will allow the freewheeling you are getting, so that is acting normally. Bobcat had corrected the problem of the rolls when stalls by having a rpm sensor on the engine feed to the brake controller so it would only be released if engine was running. The parking brake is located under the cab. you should see a gold colored solenoid coil that bolts to the chaincase cover. Remove the coil and see what is there for parts or what is missing. I don't have a pic of it but someone in the forum may? You should find a plunger for the solenoid, a few springs, a rectangular 'locking block' that will engage with the 2 discs you should see inside the case. I'm sure that something is either stuck/damaged/missing inside the brake mechanism.