The Rear brake calipers from Hell

[Cleve]

On one of our new rear IRS 2015 mustang 11.65 calipers, the piston hasn't retracked in far enough to give us the proper distance between the pads for it to fit over the brake disk. We have tried to push it in with considerable force and it wont budge. Any suggestions on how to push it in would be appreciated. We have actuated the spring loaded emergency brake lever using a screw driver and the piston does extend itself about 1/16" but still wont retract in enough .
Thanks you

[DesertSn8ke]

I haven't messed with these calipers specifically, but I believe some calipers have pistons that have to be rotated to retract them, and can't be pushed in. Maybe that's the case here? A mustang maintenance manual would say for sure. I'm sure someone with direct experience with these will chime in.

[BEAR-AvHistory]

Pistons that need to be rotated to retract will have 4 notches in the outer edge of the face. You can get an adaptor to fit them at the auto store that will fit a ratchet wrench.

Good plan is to fully retract then on a bench as it can be a ***** to do on the car.

[phileas_fogg]

As BEAR says, if the pistons have to be rotated to retract, you'll need a tool like this: https://www.amazon.com/Lisle-28600-Disc-Brake-Piston/dp/B0002SQUFY/ref=sr_1_2?crid=77IYA63Y78Q2&keywords=brake%2Btool%2Bcube&qid=1690316481&sprefix=brake%2Btool%2Bcube%2Caps%2C70&sr=8-2&th=1.

Don't be surprised if you have to grind off the pegs opposite the ones that fit your pistons (it's no big deal, 'cuz the pegs you grind off don't fit your brakes anyway!).


John

[gbranham]

Have you filled the brake system with fluid yet? If yes, you might be fighting hudraulics...in such a case, loosen the bleeder valve. If you haven't filled the system yet, (and assuming it's not a thread-in type piston), then perhaps the piston is cockeyed in it's bore from trying to retract it with uneven force. I assume you've tried a caliper piston compression tool, using an old pad (or a new one)? Just spitballing without more info...

[Cleve]

Thanks for all the help. Modified a tool to rotate piston. Was able to mount brake pads and caliper. We good to go.

[michael everson]

They are a threaded piston.
Mike