Inboard Rear Brake Insights

One of the most unusual aspects of the Jaguar XJ6 Series 1 through Series 3 sedans and the XJ-S coupes is the inboard rear brake setup. On these cars, the rear brake calipers and rotors are not located out by the rear wheels, as they are on 99% of all cars, but are located alongside the differential. This arrangement is more usually found on pure racing vehicles, where it is frequently used in order to lower the car's unsprung weight (i.e. weight not supported by the car's suspension). In general, the lower the ratio of unsprung to sprung weight, the better the suspension will operate.

The inboard brake arrangement brings with it a number of considerations in servicing and maintenance. First of all, this design makes it very difficult to replace the rear rotors, in most cases requiring the dropping of the rear subframe to accomplish rear rotor replacement. As this is a great deal more work than rear brake rotor replacement on an outboard brake vehicle, it is important to keep an eye on the rear brake pad thickness to prevent going metal to metal and ruining a rear rotor. On an inboard brake Jaguar, it is always better to toss rear pads with a bit of lining left than to replace one's rear rotors.

Second, the placement of the rear brake calipers puts them in close proximity to the exhaust pipes as they kick up over the rear axles. This, along with the lesser airflow which exists under the car (compared to calipers in the outboard position) and the normal heat generated by the brakes in operation, places an elevated heat load on the rear calipers. This in turn affects the service life of the rubber seals in the brake calipers. They harden and the calipers begins to leak brake fluid from around the caliper pistons. This condition should be suspected if inspection of the rear caliper dust boots reveals that they are badly deteriorated and falling apart. Additionally, rear brake pad replacement, and the requirement to push the caliper pistons back to allow insertion of the thicker new pads, will frequently cause brake fluid leaks to occur even if the calipers were sealing before. The only solution to this problem is to remove and rebuild the rear calipers (or replace them with rebuilt units). Keep in mind that if brake fluid changes were not done per Jaguar recommendations you may find quite of bit of internal corrosion in the calipers, making them difficult or impossible to rebuild.

Third, the handbrake calipers, which are separate mechanical calipers mounted above the hydraulic service calipers, are frequently seized due to corrosion and deterioration/hardening of the factory grease. These must be removed to enable removal of the service calipers and it is a good idea to disassemble them to clean and to free up. Glass-beading and relubing will normally restore the calipers and self-adjusters to proper operation.

Fourth, it is always a good idea to replace the rubber brake hoses when they are more than ten years or so old. Old brake flex hoses generally look fine externally but over time the internal passage tends to swell closed and act as a one-way valve. Because most disc brake system do not have positive pad retraction mechanisms to pull the pads away from the rotors after brake applications, this can cause the brakes pads to drag lightly on the rotors resulting in excessive wear and rotor overheating. (Lateral run-out of the rotor and seal hysteresis account for brake pad knock-back in most disc brake designs.)

The above information also applies in general to XKEs, Mk 10s, and 3.8S Jaguars, which use basically the same inboard brake independent rear suspension.