Brake caliper rebuild

Hi Jagies !
I want to rebuild my front brake calipers and would need some help.
I do not want to fit new seals only but zinc-plate the calipers. I think I have to use compressed air (if sufficient) to push the 3 cylinders out and will have to separate the two parts of the caliper and reinstall with new seals in between ? Is there anything I can do wrong or is the job quite foolproof ?

Thx, Tom
p.s: I did read some old articles but could not find the right answers

If the compressed air is insufficient then the next step is to screw the end of a grease gun into the supply port and use that.
Grease being incompressible will move most pistons unless totally rusted in. Once you have one moving (path of least resistance) you will need to restrain that one and rinse and repeat for the remainder.

Thx Robin,
I will try this tomorrow (my compressor is too loud now at 11pm) and will use grease as you advise. Will it be necessary to split the two halves or should i leave them together ? I hope I can reuse the cylinders after I have cleaned them thoroughly.

Probably a case of making a decision once the pistons are out. I don’t have any personal experience with the specific calipers you are working on, just using information garnered over the years working in my workshop on various makes of cars, along with my own ‘S’ XJ40 and XJR cars

Before using the compressed air put a bit of wood about the thickness of your disc between the pistons. This will stop you blowing out one piston completely leaving the others firmly stuck in place.

Otherwise the job is simple.

The scuttlebutt is that separating the caliper halves is a no no and should never be done. I can’t work out the logic of this, and always split the calipers to replace the channel seals as well, which can be bought from any brake place once you actually know what to ask for.

Depending on your degree of paranoia you might decide to replace the bolts that hold the calipers together with new grade 8 ones. I sometimes do, sometimes don’t. Do NOT use stainless steel bolts, only proper grade 8 steel. If they are black you can plate them safely.

Don’t sandblast the mating surfaces of the 2 halves and mark the calipers prior to disassembly so that you reassemble each pair correctly.

Sandblasting inside the caliper is otherwise fine as the important hydraulic surface is on the piston. Inspect the piston surfaces carefully. Tiny imperfections can be polished out if they are not directly under the seal surface, but if you find anything worse than that you should replace the affected piston(s).

Plate the calipers halves separately. The few microns of thickness of zinc will not adversely affect fit. To be honest though, I have gone to painting calipers as the zinc finish quickly deteriorates over time. There is no reason of course why you could not zinc plate them and wait and see and paint them if you are unhappy. You could also clear coat them with an appropriate paint after the plating.

Plate the bleed nipples and other mounting hardware at the same time. I have actually plated the short caliper lines in the past too but some disagree with this. The car I did that on is still in regular use nearly 10 years later without incident. These days I usually make up new pipes whenever I do a major caliper overhaul.

Getting the piston to go through the seal for the first time can be difficult. Use lots of rubber grease.

You may as well get your discs skimmed at the same time. Have fun.

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Hello Top,
The Seals run on the Piston OD with S2 and S3 calipers, unlike S1 where the Seals run on the Cylinder Bore. Accordingly, the Cylinders of the Calipers have to be unbelievably rusted and damaged to not be able to be reused.

There are Square Section “O” rings used between the two halves of the Calipers and the Spacer between them. These should be replaced and are quite hard to find a supplier. There is one in the UK I know of and can supply their contact details if you require it.


Bill .

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Hello Andrew,
I’m in your camp with regards to this point. I work on the principle that the caliper halves were assembled by a human and therefore, I’m part qualified.



Quite rightly too Bill.

How’s sunny Ballarat today? I’m just about to book a truck to pick my 3.8 coupe up from the trim shop and do the final fettling before getting her on the road here!

But what would be the benefit halving them ? Couldn’t I just have them zinc-plated as they are or maybe use silicone sealant instead of the hard to get rings ?

“… degree of paranoia …” :rofl::joy:

p.s: found this:
Recommendation to torque the 7/16″ diameter bolts to 70 lb-ft and the 3/8″ diameter bolts to 40 lb-ft. Work from the outside in and alternating sides each time, so torque the outer bolt on one side then the outer bolt on the other, followed by the inner bolt next to the first outer bolt and finally the remaining inner bolt.

Hello Andrew,
The Sun is peeking through, but its cold. If it weren’t for Global Warming it would be bloody cold.

Best regards,


Thx Bill ! Could you please PM the supplier for those … just in case !

Hello Tom,
Yes you could have them plated without dismantling them completely, but in my opinion, its a job half done and one that is not very difficult to do. A set of XJS front calipers, that had been fitted to an S3 E, that I just finished rebuilding, had to have the crud removed from the various galleries with a twist drill. By splitting the calipers you can be assured that once thoroughly cleaned, they are as they were before their initial assembly.



Hello Tom,
I’ll have to look it up, but I’ll post it here for others to see as well.

Where are you, as I’ve ordered a few dozen; more than I need. As long as its not a case of shipping Coal to Newcastle, I can supply you some.



Hi Bill !

I found a source in the US but I am living in Europe/Austria. At least now I know what I have to look for and most opinions seem to favor the splitting to be able to clean the parts … So I will start taking the cylinders out. Why do I have to use new Bolts - are they self fixing and destroyed when unbolted ?

Cross Feed Seal for Girling Calipers

Click for a larger picture of Cross Feed Seal for Girling Calipers

Caliper manufacturers and knowledgeable racing mechanics recommend against splitting the caliper halves when rebuilding. If you do split the caliper, the cross feed seal must be replaced prior to reassembly.

This seal measures 0.480" OD x 0.075" thick to fit Girling 14LF and 12SP calipers. Sold individually.

p.s: I also found them at Moss Motors with part# 583-820 and will call SNG if they can also source it.

Hello Tom,
No, the bolts aren’t destroyed, but if you’re going to the trouble of having the calipers zinc plated, I suspect that you will have the bolts plated as well. Two different sizes of bolts are used that are available off the shelf at most Nut and Bolt supply business, already zinc plated. They are not expensive and having your existing bolts plated will probably be a similar price, or more, as they are high tensile bolts and will have to be baked, post plating, to circumvent Hydrogen Embrittlement. You should mention to the plating company that the bolts are Grade 8, High Tensile bolts, to ensure they are managed correctly post plating, if you decide to use your original bolts and have them plated.

Following are the contact details of a company in the UK where the Square Section “O” are available.

Bigg Red Ltd
The Workshop
Weir Lane

Tel: 01905428793



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Paranoia. Maybe.

Brakes HAVE to work, or you die.

Bolts are cheap, easily available and just about impossible to assess for integrity.

Hi guys,
Well the pistons seem to be too worn and I have now decided to purchase new calipers from SNG. I will lay the the original calipers aside and rebuild them some other day maybe.
I plan not to sandblast them but have them vapor cleaned just as they are and see the result. They were deeply covered with bitumen like protection but underneath they seem to look brand new. When I rebuild the IRS I will stay with the original ones and ordered pistons already.
I am doing this all in my unique “Great and Unmatched Wisdom” as a “Full-God-E-Type Driver” :rofl::rofl::rofl:

Yeah, ‘seem to be’.

If you ever have to chair a meeting, the big one makes a nice gavel.

Can also be used over the top of the post of a bottle jack to spread the load.

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Aww that’l buff out :innocent:

Tom – Vapor Cleaning or Vapor Blasting will NOT clean parts. Vapor blasting dopes its thing after the party is cleaned (media blasted?).