Disc Brake Pistons

For some time I’ve been trying to remove the pistons from my 1960 MK IX brake cylinders. The car is a garage find and I’m sure it’s been over 30 years untouched. I spend hours trying to pry the pistons out. A while back someone posted a trick to push them out with a grease gun. So, I modified some brake line fittings – one with a zerc (sic) fitting and the other with a plugged fitting (J-B Weld). Leaving the plugged fitting off, I started pumping. After about 20 pumps grease began exiting the unplugged hole. I then screwed in the plugged fitting and started pumping again. Then, like the Phoenix rising from the ashes, the piston began moving! Happy dance. Thanks to whomever posted the trick. Thanks for the early Christmas present.


Thats how I’ve done them for awhile now. Works great.

You can also just use oil or water in the grease gun. Less clean up.

Compressed air works well.

I had no luck with compressed air at 100 psi. A grease gun generates a lot more force plus the grease is not compressible. The rubber rings were rusted in place. I was able to carefully polish the cylinders and don’t think they need to be sleeved.

I confirm that compressed air is useless if it’s properly seized. Filling the caliper with oil or water (…liquid…) and then the grease gun does it.

Usually the grease hose even fits the imperial threads and seals well enough.

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Yes, I’ve no doubt you’re right. The last one I did in fact required about 180 PSI IIRC (I use dry nitrogen from a cylinder). And you have to make sure the piston will hit something soft, like wood, when pops. Agreed, grease is better overall, but you can try air (or N2) first to see if you can avoid the mess.

You should refrain from that. As a compressible gas, it is very dangerous to use air for this sort of thing. You run the risk of making the freed piston a lethal projectile. Whenever you are doing pressure testing for ASME/ANSI code pressure vessels, boilers or piping it is always done with something non compressible and non hazardous, most usually water. For this reason, it is called hydrostatic testing. You can fill the vessel with water, seal it, then use a small pump to raise the pressure to the desired pressure. Once all air is gone, this happens very quickly.


Of course, but for regular piston servicing the 11 bars don’t do too much damage, it would surely knock you out and cause a lot of damage to the target but a vise contains that safely.

For anything bigger than the few ml it’s very smart not to use air.

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I do have a follow-up question for someone more knowledgeable than I. There is a pin in the center of the disc brake cylinder that fits into the center of the piston. I note that it is an extremely tight fit and there is some sort of fitting inside the piston that allows it to rotate on the pin. (1) Is it supposed to be that tight and (2) what purpose does the pin serve?

Short answer to (1) is that it is supposed to be that tight and (2) it serves to retract the brake pads upon release of hydraulic pressure by engaging a spring-loaded clamping device located within the piston.

Much longer answers here although Compensation Circuit was a slight misnomer.

This whole thread would make more sense (and probably get more responses) if it were placed in the appropriate Jaguar model category, rather than the XK-Engine category. Maybe the original poster can move it, or perhaps it requires an Admin to do so.


Any “regular” member can move it. I’ll be happy to do so should the OP want me to. Where would it go? Saloons?

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If by any chance you need any calipers, I have 7 rears and 6 fronts I’d let go cheap.

I’ve moved it to the saloon forum.

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Thank you for answering. Apparently I posted on the wrong forum and will try to move it to Saloon.

One of the moderators, Robin, has already done that for you.

Yes, I saw that. Thanks!

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