So after putting in a diff from an XJS of unkown origin and taking a gamble on the condition of the brakes (the pads looked fine), I lost that bet!

Ho hum, that will teach me!

After a quick test run one of the rear discs was scorching hot (300 degrees C).

So diff back out again (getting good at it now!)

Looking at the calipers I noticed that the retaining pins were bent and the pads were certainly not free to move in the caliper.

There are signs that the chrome has given up on the pistons where they are exposed to the elements.

The seals look OK though.

Would bent pins cause the brakes to seize or just general lack of use maybe due to hanging about in a scrapyard?

If I want to pop the pistons out, do I need to split the caliper? I assume so but then I read something about not doing that?

 

WEEEELL.

Its out, so I suggest do it right and repeat in 20+ years.

Lack of all sorts by my one eye.

I would also be replacing the output shaft seals, assuming its a Salisbury diff (got a drain plug, the Dana no got drain plug).

Splitting the rears is NOT an issue, as they use a bridge pipe for fluid. The fronts are a NO NO, unless you have the specific square section o/ring port seals.

 

If you were in the States I’d suggest getting reman calipers from Rock Auto. There may be a source for them in the UK, I’m just not sure what it is.

 

No need to split the caliper to remove the pistons. The bent/rusty pins WILL lock the pads into the disc. Rebuild the calipers. New pins are easily available. Do NOT take do half a job on Jaguar mechanics or electrics, or as you have found, you will regret it!
Refurbed calipers are also easily available but are about 100 quid each.

 

Thanks everyone.

Twas my own fault to not do this weeks ago while the diff was sat in the garage waiting to go in, but hey ho. I was way too eager to see how the 2.88:1 diff ran in comparison to the 4.55:1. The motor ticks along nice at 60mph in third gear. No more reving the nuts off it to keep up with anything other than a tractor... And now it will run even better without the rear brakes on!

How DO you bend a retaining pin????

 

Hammering it out when it's rusted and seized in situ?

Paul

 

Oddly the pins came out easy...

 

Pistons out. Rust pretty bad, creeping down the sides!

Any tricks to getting new pistons back in, without splitting the caliper?

 

OOPS.

Plenty of brake fluid as a lube, juggle them around, slow as you go. They will go in, or you will concede and split them.

No special art to it, just time and care not to damage the seal.

 

Hi Rich333

How to rebuild the Rear Calipers on an XJS V12

How to rebuild the Rear Calipers on an XJS V12

 

If you have not already put these back together, go ahead and split the caliper. It will make it much easier to remove all the rust that caused it to seize in the first place. They rarely rust behind the piston seal, they rus between the outer dust seal and the inner piston seal. I split the caliper and used a brush on a Dremel type tool to remove all rust, and used Girling red rubber grease in that area to hopefully, prevent any rust in my lifetime.

 

So the rebuild seemed to go ok, put them back in with new seals and pistons.

BUT one of the pistons is exhibiting an odd behaviour. When the pressure is taken off, the piston moves back into the caliper by about 1-2mm. I can't get my head around why it would do this.

It obviously means that next time you apply the brake, there is too much pedal travel as the piston has to move 1-2mm before it even touched the pad!

I took the caliper out again and popped out the piston. It looks fine as does the seal. I wondered if it had twisted or got pinched. I've checked it out of the car with compressed air and it does the same thing.

It not the dust boot as I took that off and it still does it.

Video here:

 

Did you lube the seal, piston, and bore with fresh brake fluid? Or you can use "Red rubber grease" especially made for rebuilding calipers and lubing seals and pistons.

 

Yes, I used fresh brake fuild. The piston did this when it was on the car with brake fluid in too...

 

The only thing I can think of that might cause that would be a piston that isn't as smooth as it should be, and is grabbing the seal instead of sliding. The surface of the piston should be almost mirror smooth.
Also, pull the seal back out and make sure it's groove in the caliper doesn't have some sort of junk behind it.

​

 

Good call, but already tried that. I took the seal out and checked the groove. There was some muck in there but nothing large. I went round it with a pick and made sure the corners were squeaky clean.

New pistons are nice and smooth. Without the seal in, they slide in nice and sink slowly as the air comes out.

 

Is that ones surface as smooth as it's mate in the other half of the caliper? Try swapping them and see if the issue follows the piston or the caliper half.