Hi all. I have a replica AC Cobra, employing Jaguar XJ6 front suspension and Early S-type (1960's) rear . From day one when I first registered this car in 1994 I've had nothing but problems with the park brake. I put new pads in and regularly need to adjust the cable for the park brake to partially work. The fact is, I can't get get the car to hold, even on a gentle slope. Are these brakes useless? I would think that they worked as they should in the original cars and mine is about two thirds that weight. Any suggestions would be gratefully acknowledged. Thank you. Bill Maslin

 

hi,
I'm not a Jaguar brakes expert, just basic experience, but the MK-2 and S type rear brake calipers system are not the same. The S type (and 420, XJ-6, MK-10), rear brakes are a carryover design from the E-type's Independent Rear Suspension (IRS).

In the S type and XJ-6, the rear brakes are sufficient to stop the car in an emergency if the front brakes should fail and you have enough distance to avoid hitting another car, so they are not exactly useless, But in both cars, the braking power is biased towards the front calipers/rotors, the rears "help" in braking but the "brunt" of braking power is at the front.

I can only guess that the problems are, your rear calipers are sticking or your system is missing some component.

 

There is nothing really wrong with the handbrake on these cars, as it has been identical from the E-type/S-type/Mark10s of the early 60s, to the XJ6 Series 3 end-of-production in 1987. These handbrakes can pass normal British "MOT tests", a mandatory roadworthiness test and certificate and a certain percentage efficiency has to be achieved.

The main problem is that the rear brakes being inboard discs they get neglected. There are separate calipers for the handbrake located on the main caliper. These have an automatic adjustment mechanism in the actuation lever. This commonly seizes up or wears over the years so it no longer adjusts. Adjustment is not achieved via the cable, and attempts to do so will fail. The cable adjustment is there just to take up slack on the cable. Other things that go wrong are the caliper locating pins rust up and the calipers then cannot swing properly. The only cure is to drop the suspension subframe, remove the calipers, and rebuild or replace with new or service exchange items. Working on the rear brakes "in situ" on a Jaguar is close to going to Hell and back !!

Of course on a Mark 2 they are outboard, so much easier to service and repair.

 

Jose
Thanks very much for your response. Yes, mine is the independent rear end. I will follow up on your advice and check the calipers for worn parts, although I fully refurbished them when I built the car and have only travelled about 30,000 kms
Bill

 

Hi Fraser. Thanks for your useful reply. I will check the operation of the calipers as you suggest. Fortunately my rear end is out of the cradle so I should be able to get to things in situ. (famous last words!)
Bill Maslin

 

I would second that. theres not a lot to go wrong mechanicaly but their position makes them a prime candidate for the elements to do their worst, as is usuall.
barry

 

Bill,
Fraser is right on the money.

 

The handbrake operating lever has a little adjustment mechanism in it that relies on a spring-loaded pawl moving over a toothed adjuster nut. The nust has to be well lubricated as the pawl has to move it a notch when clearance needs taking up. Now bear in mind it's a very long time since I dissembled and reassembled one of these, (actually on my Mark 2 in about 1986 !!). What I do remember is the swivel pins need a good lubeing with high-temp grease. There is absolutely NO lubrication facility for normal servicing, other than stripping, greasing and reassembly. A real PITA. !!Notwithstanding this, the handbrake can keep going for a long time without this lubing, but the other key lubing is the adjustment nut. The clearance gradually increases until the pawl is able to move over a tooth on the adjustment nut. When the handbrake is release, the pawl in the lever is supposed to move back and push the nut round a bit as it does so, thus taking up the clearance. What does happen is the nut seizes, the pawl comes back without taking the nut with it, and wears a notch in the nut tooth so it well never adjust again, it has to be replaced. That is why one must always replace these nuts, if refurbing the rear brakes.

Very late handbrakes have some small differences in the parts but the design is the same.

Here's a website with the parts shown apart and assembled. If you look carefully, you can see the circular adjustment nut with its machined grooves (or teeth) sitting upright to the left of the first photo. You can just about see the pawl mechanism sticking out from behind the black cover.

CLASSIC JAGUAR RECONDITIONED REAR HAND BRAKE CALIPERS on Car And Classic UK [C164948]