Brake caliper piston won't go in
 

I attempted to do the rear pads on our CM yesterday and had a first...the piston wouldn't go in. All the others I've done I just had to crank in by hand using the Harbor Freight brake tool. This one I was using a big wrench over the handle for extra leverage and even then I only got it in about 1/2 way. I had to reassemble with the old pads. This was with the bleeder open after initially attempting with it closed (which is how I've done all the others). I didn't have any issue when I did the fronts a few months ago.

One interesting thing...I didn't get any fluid coming out of the bleeder.

Any ideas before I either take it to The Man or get a new caliper?

Are you rotating the piston while you compress it?

The caliper tool allows it to rotate while compressing, but I am not trying to make it rotate in any way. Not sure how I could.

It did rotate about a half turn in the amount I was able to compress it.

the rear caliper pistons screw in ... it has to do with the emergency/parking brake

ecstuning.com and others sell a special tool for the rear calipers

That's an equivalent set to the harbor freight one that I'm using. They just push, but allow the piston to turn. They don't force a turn. Pretty sure I'm using the right tool.

You need to turn the piston clockwise to compress it back into the caliper. Only way to get the rears to retract--it is the parking brake. The parking brake screws the piston in --ratchet action to activate the piston.
Your tool is the same as the ECS tool. 1-Attach it to the piston with the proper adapter 2-turn the piston with the tool clockwise 3-use a wrench (or simply by hand ) turn the hex portion counter clockwise to keep it in contact with the outer caliper 4-repeat until the piston is fully retracted. Sometimes it a little difficult to turn the tool handle if the piston is in bad shape.
Yes--to open the bleeder screw.

The above information is correct, they retract by rotation.

It is a slow and deliberate process, can take up to 10 minutes or more to retract them turning them.

Motor on!

Thanks guys. I think my issue was cranking the handle instead of the hex portion. I'll try it this weekend and report back.

You need to turn both--handle clockwise and hex counter clockwise, as the piston rotates and retracts ... be sure the ebrake is not activated (goes without saying, but!!!)

Well I figured it out but hosed up the caliper boot while figuring all this out and now it's leaking. I probably should just buy a remanufactured one instead of trying to replace the boot. I'm surprised these are only $60.

https://www.rockauto.com/catalog/x,c...,parttype,1704

The key thing I missed is that it retracts while turning. I thought it needed to be allowed to turn while being pushed in. The Harbor Freight brake tool I have was missing anything that kept the end piece in alignment with the handle, so there was no turning happening. A small screw in the alignment hole fixed that.

Also, the pads that AutoZone sold me are entirely too small...I'd guess only 2/3 the surface area of the OEMs. Any suggestions for pads?

If it's leaking, then it is more than the boot--probably the seal or bad piston. Replace the caliper.
They probably sold you the wrong pads. Although the Cooper and the S use the same pad!! do you have JCW rear brakes?