Front Brake caliper Piston Stuck??
 

Hi,

Have in mind to refurbish all the front suspension on my 940 at some point in the near future, so with great suggestions from the forum I sourced from Luke 360Beast some second hand struts/arms and front calipers, so I could refurbish these at my leisure, and then do straight swop.

Thought I'd start with the 'simple' task of cleaning up the calipers, and as they are bare units, I'm really struggling in removing the piston, had several days of soaking the piston in penetrating fluid Aerokroil, and also tried most recently 3 days in ATF/Acetone.

Obviously if still on vehicle, I'd normally use brake fluid pressure to pump the piston out, but this isn't really an option.

Tried air compressor using small piece of rubber to protect thread of the female hose entry, several attempts but the piston hasnt budged at all.

Some Youtube suggestions about using grease gun??

Any thoughts or suggestions would be most appreciated??

Cheers
Steve

Sounds counter-intuitive Steve but try getting the pistons back into the calipers first. If they move that way, they should then move the other way and a large G-clamp can be used to do that.
You may have to use a combination of air pressure on the inlet ports and G-Clamp to repeatedly push in/out until you get the full range of movement.

Grease gun in theory would work or if you happen to have a spare master cylinder and some brake pipe, make up a jig so you can use hydraulic pressure on it.

*** EDIT *** Alternatively electrolytically derust it first and then get the piston out easily.

https://www.mig-welding.co.uk/forum/...leaning.12686/

It only costs pennies to try and if it works, much of the hard work of derusting the outside of the caliper is already done for you too! :thumbs_up:

Grease gun method will work but it's messy as hell and wasteful. If you've got a relatively tame local mechanic I'd get them to do it. literally a 2 minute job with workshop compressed air

Hi Dave,

Thanks so much for suggestions, afraid I haven't yet built up a supply of old bits such as hoses etc for me to set up a temporary system to push it out.

I did ponder pressing the piston in, but bottled it.

Caliper has had 3 rounds of electrolysis with penetrating fluid then used around piston sitting overnight.

You're so right, no easier way to de-rust an item, the amount of scale and general crud that comes off is amazing. I had this project in mind when I decided to buy the 940, so had got hold of an old 'dumb' charger.

Your suggestion regarding pressing in, makes sense to me now, think I will inject some penetrating oil through the hose inlet so that it can work from inside the caliper bore/piston, fortunately I use some very thin syringes and needles for some of my woodwork projects, so I should be able to get teh bore filled up with fluid.

Luke kindly pressed out the old bushes in the arms for me, and glad he did, as he had a fight with one of them, even with his nice new press.

Calipers have come up well, so struts and arms should clean up really well with electrolysis.

Will update regarding progress on calipers.

Cheers
Steve

Bung it in a vice and get two flat head screwdrivers into the groove on the piston and lever the piston out. As Dave said it can work if you push it in a bit first

Hi Mark,

Thanks for the levering out option, will keep this one as last resort as its not time critical, so its sitting with some penetrating fluid behind the piston, and i'll try Dave's suggestion tomorrow to press it in, as once it moves a bit, I can then use my air compressor to hopefully push it back out.

If all that fails, then levering out can be a fallback option.

Oh by the way, and not really a surprise, new correct estate spec rear springs changed in this morning, sitting much better now...... some almost new saloon spec springs to go on for sale section, what a numpty.......

Cheers,
Steve

NO...do not try levering the piston it will break the rim...ask me how I know !!

In theory, the piston doesn't make contact with the bore of the caliper, the piston rubber seal is the only part that does, so what goes in should come out, air pressure usually does the trick, if you can get a good air tight connection to the fluid inlet, or the bleed nipple and seal the inlet it might work using just a footpump, but an air line would be better.
Careful though, it will come out with great force.
As suggested, your local garage might do it for you

I've levered loads of pistons out without breaking anything

I must have been unlucky then, but I wouldn't take the chance if I wanted to re-use the piston

I assume if the piston is that stuck you are rebuilding with new pistons anyway. Or probably should