Rear brake pad fit after caliper rebuild

I just reinstalled my rear calipers after rebuilding them during which I separated the caliper halves. (it would be very hard to rebuild them without doing this IMO) Got them both back in the car and put in my new brake pads and pins, Right side was a breeze. Left side -uh-uh. The first pin would always go all the way in. The second one would not go into the receiving side. I tried swapping pads, pins, pin directions, top or bottom pin first, and every time that last hole could not be made. I finally took the pads and opened up the holes a bit with a Dremel. The holes are already elongated tangent to rotation out of the box. I opened them up in the radial direction. They installed easily after that. I realized at some point that the caliper design requires eight points to align within a narrow tolerance and that alignment could be upset by the caliper halves being slightly misaligned. When I bolted them together before, I noticed that there were no pins or locating features, so I assumed (ha-ha) that it didn’t matter. The alignment is “controlled” by the clearance holes in the half that the bolts pass through. There’s not much slop but apparently enough to cause assembly problems. If I ever do this again, I will have the brake pads and pins installed as I tighten the caliper halves together. I don’t think this will matter one iota in brake performance. I’m just passing this along as (hopefully useful)information.


Thanks for sharing, Tony,

just as you probably have spent a couple of hours over the problem, anyone else would - maybe without finding a solution! Do we have an “info of the month” laurel, btw?

Good luck


75 XJ6L 4.2 auto (UK spec)

I’m glad you were able to put a face on what the Repair Manual has to say about separating the calipers: “CAUTION: Do not under any circumstance remove the four setbolts securing the two halves of the caliper together.”
Without any repeatable locating means when the factory assembles them, I would imagine that they are put together on a basic jig and then machined to final spec.


You have to to renew the pistons and seals

Good info from the OP though…never had any trouble myself, I generally scratch a mark across the halves

…which can be done without separating the caliper halves, Tony…:slight_smile:

xj6 85 Sov Europe (UK/NZ)


Hi Frank,

I suppose I can see how it can be done to extract the 2nd piston without splitting the caliper, but it would be more difficult & time consuming than splitting the calipers.

In addition, my “brake guru” who has been a brake specialist & proprietor for 40+ yrs advised me this is how he does them, the guys in our Jag club also do them that way

A blast of compressed air and all pistons, in both my front and rear calipers, popped out right away, job done in ten minutes.
I see no need what-so-ever to separate the calipers unless the pistons are totally rusted and fused solid and you need a big fat vice and a big fat hammer (and probably new calipers).

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That must have been extremely nice for you, but indeed I have had pistons very reluctant to come out, especially both at once, just from age, scum & piston chrome pitting .

In the instance of a 420G front caliper, (for instance), they are very scarce and expensive, the seal is in the caliper, so they can be salvaged if the old piston can be extracted

My method gets them out, I have refined the process, no vice, but hammer yes…tap them back down into the bores to start the process

Yes it was!

The other trick I’ve heard somewhere is to use a grease gun.
Or even better, extract the pistons before removing the brake lines with the use of the brake pedal.

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I use the front DS brake line to do this

I split the calipers, soak them in penetrant, knock the pistons back down with a brass drift, then use the brakes to work the piston out, with a G-clap to prevent it flying out

Brake fluid pressure is very high…hundreds of psi…they just laugh at compressed air

It has been known to take several attempts to succeed

(example salvage vehicles…every possible effort should be made to save 420G calipers)

I’m a bit bewildered, Tony…

While extracting pistons sometimes has required use of devious methods - even sizable pliers; I have never experienced any problems fitting them back in. The bores and pistons cleaned out, and sauced in brake fluid - they have always slid in with no problems…

After all, the pistons are supposed to move ‘freely’ - and retracting minutely by the seal elasticity.

Splitting the calipers is a last resort - the channels inside is fairly fine and require some precision. That professionals routinely split the calipers for overhaul, should not encourage splitting - against specific warnings not to…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

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cant have that…

no probs putting the pistons back in…they will be new ( or sleeved)

The caliper splitting is required if the pistons wont come out.
I have encountered this often, as have others

If one encounters this issue, tapping the old piston down into the bore breaks the seizure, then hydrualic pressure is used to raise it up. Rarely does it work with just one effort, usually needs to be knocked down a few times.

rear pistons are especially prone to seizing

As I understand, the standard practice would be to throw them away, and order a new caliper/piston set.

They can also be dismantled this way, and the caliper sleeved in SS, which is more economical and long-lasting than new ones

Its just 2 machined surfaces, I have used a torque wrench to do them up evenly

I think the issue of caliper splitting has been covered before on JL, and considered acceptable, at least by some…but I could be wrong

This is a finer point I hadn’t considered. -that there could be misalignment of internal fluid passages between the halves on reassembly; could it also introduce a leak in the interface? That would suck! Mine have an external pipe to link the halves so no worries there. I got the pistons out with a tire pump (after squeezing the seized pistons in with pliers) before I separated the halves. I only separated them to have better access for cleaning off surface corrosion with a Dremel wire wheel.

Some years ago, the time came to fix the weeping rear calipers on my lump. After much readin here and deliberation and consideration of past practice on other critters. Solved.

I bought a pair of refurbed loaded calipers from Rockauto and hired a new shop to install them. About 800 all in… Carl

…‘acceptable’ is in the eye of the beholder, Tony - my point was just that splitting the calipers is not necessary, nor advised for a rebuild.

It doesn’t mean instant disaster if the caliper is split - just that you should know exactly what you are doing…:slight_smile:

As an aside. Before I start doing caliper overhaul, and with calipers in-car, I ensure that the pistons are movable - making extraction easier. Using the car brakes and worn out pads - and better leverage to move the pistons in with the calipers solidly fixed… Admittedly, a pain in the neck at the Jaguar rear…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

I did this job many years ago on my '74 XJ12 S-II and some of my pistons were rusted and stuck so there was no way to get them out without splitting the calipers. I remember reading this “caution do not separate” statement in the manual, so I asked a co-worker who was a Corvette expert, and he said there are just rubber seals in there and he splits them all the time for rebuilds. So I did it, got the old pistons out with penetrating oil and heat and big channel locks, had no difficulty after that. The bolts were close fitting, so I had no trouble with reassembly and pad pins.