Transmission brakes use drum brakes, rather than disc brakes, as they are intended as a static parking brake, rather than a high performance dynamic brake. Drum brakes allow simpler adjustment with cable-actuated hand lever mechanisms. The brake is mounted to the rear output shaft of the transfer box.
As the transmission brake is mounted inboard of the final drive and its reduction gearing, the brake rotates relatively faster, but with less torque, than a wheel brake. The apparently undersized transmission brake thus has more holding ability than its small size might suggest, but is less suitable for driving loads.
One advantage of a transmission brake is that it locks the entire drive-train, including all four wheels of four wheel drive vehicle. However any differential action, either within an axle or front-to-back on an all wheel drive (permanent 4×4) vehicle can still allow movement. A second advantage is that they remove the need to provide cable connections to the wheel brakes, on off-road vehicles where such may be prone to damage.
Note: The HF1A transfer case fitted to the FZJ and HDJ models may have an oil pump in the rear housing, if this is the case this kit will not fit your vehicle.