Release the piston

Magnus Karlsson
Boras, Sweden
Edited by Reid Trummel

Stuck pistons in the brake calipers can sometimes become really stuck. The normal method to "unstuck" them is to blow compressed air into either one of the holes for the bleed nipple or into the brake pipe nipple. To do this, first make sure that the other hole (the bleed nipple or the brake pipe nipple, whichever one you're not using to blow in the compressed air) is blocked. Then insert a piece of wood between the pistons in order to dampen the impact when the piston releases.

Piston that wouldn´t let go eventhough grease was used through the bleed nipple hole.

If the compressed air method is not successful, try the grease gun method. It works along the same principles, but you fit a pointed end to your grease gun and hold that pointed end very tight to one of the brake fluid holes in the caliper. This method will exert a much higher pressure on the pistons than the compressed air method will. However, if the pistons really are stuck, chances are that you will not beable to keep the pointed end firmly enough against the hole and thus the grease will leak out.

Tapping a hole in the piston in order to fit a grease nipple.

Now is the time to become "radical." First you split the calipers. This is something that many people are afraid to do since it is stated in Girling manuals that this is strictly forbidden. However, Girling gives no reason as to why this practice should be avoided. I cannot take any responsibility for what can happen if someone reads this article and decides to split his calipers, but I can say that I have done it with many calipers and never had a problem. Myself, I prefer this practice to scrapping calipers with stuck pistons. It also gives one the opportunity to change the seal between the caliper halves, the condition of which can leave a lot to be desired after 40+ years. It is important to use a rectangular-section O-ring (flat, like metal washer) for this as it has been manufactured to function in a brake fluid environment. An ordinary O-ring (with a round section) may not be compatible with brake fluid, since most of them are manufactured to function only with oil and gas.
After your calipers are split, you next drill a hole through the piston with a suitable diameter drill, so that the hole can be tapped to fit a grease nipple (see photo). Then you block off the brake fluid passage by clamping the caliper half in a vice with a piece of rubber mat covering the passage. One of the caliper halves is easy to clamp in the vice. In order to clamp the other half you must use two of the caliper bolts as seen in the photo.

Caliper bolts used so that the caliper half can be clamped in the vise.

Then attach your grease gun to the nipple and start pumping it. You will find that the stuck piston comes out very easily and in a very controlled way. I have yet to experience a piston that will not release when subjected to this treatment.

After completing this operation you will of course have to replace the pistons, the seals and the dust seals. I recommend that you fit stainless pistons as they are available for both types of calipers used on Healeys.

Restored caliper half fitted with new seals and stainless piston. Please observe the correct square section O-ring, top left corner.