Front brake piston won’t push back

[Smithy]

Good evening everyone.
I had a bit of an issue with one of my front brakes. I noticed it when I changed my wheels over that one wheel wouldn't spin as well as the other. So today I stripped it down to make sure the piston was moving back and forth ok. Pads and disks look fine but the piston isn't moving well. I pumped it out to clean and copper slip but it still won't push back. I'm soaking it overnight with wd40 and will try again tomorrow night but just wanted to ask if I'm missing something? I'm using a G clamp to push it back but with no joy. Thanks everyone

[potge]

Unfortunately, I dont think you are missing something. A g-clamp or piston retract tool should be able to move it back. You can open slightly the bleed nipple to aid with removing pressure, but it should not be needed.

You most probably have a seized piston. In my case some rotation helped to break all the gunk but ultimately I had to rebuild it. So based on what you say if you are going for a DIY you can pump the piston out and then remove the caliper from the car. Or if you go for a new/refurbished caliper just swap it.

[Smithy]

Thanks @potge i will give it another go tomorrow and see how I go. Thanks for your guidance

[Joesson]

@Smithy
First of all the "copper slip" and "wd40" will have almost certainly contaminated the brake fluid and likely the rubber shroud and seal.
I believe the piston is likely past  pushing back and should be removed and cleaned/ replaced.
With caliper on the car the piston can be pushed a long way out by removing the pads and applying the brake pedal.
With caliper off the car I pushed/ knocked the piston out with a rod through the hose banjo bolt that is central in the caliper bore behind the piston. I intended to replace the front pistons but no damage was done that way.  I had previously  tried to rotate the pistons, that did damage them.
I doubt your pistons are in any better condition than mine, so you will likely need to replace it/ preferably them as both sides will be in a similar state and it is considered good practice.
You will also need a seal kit  and banjo bolt washer and to replace the brake fluid.
I used methylated spirit to clean the components. Proprietary cleaners are available.

[Smithy]

Thank you for that. I will have a try tomorrow but looks like I will be looking for some new ones.

[Smithy]

Bit of an update. I managed to get a 2nd hand set of callipers to swap over. I just need to read up on how to swap over without losing all my fluids.

[paulj]

Brake pipe clamps can keep most of the fluid in for you.  That said, when was the fluid last changed as it does attract moisture - if more than a couple of years old it would be great to flush and fill as part of your refurbishment

[Smithy]

It's old fluid for sure. Do I just let it drain out from an open hose, replace the callipers and then refill?

[Carolyn]

It's old fluid for sure. Do I just let it drain out from an open hose, replace the callipers and then refill?

A better technique is to bleed all four corners, starting with the point furthest from the master cylinder (nearside rear) open the nipple and let it drain, keeping an eye on the fluid level.  When the draining fluid starts to look clean, shut the nipple and bleed by vacuum or pedal pumping until it runs clear and bubble free.  Then do the other rear and then the fronts.

A bit of a PITA, but you'll end up with a clean system with clean  fluid.

If you push the piston out of the caliper, you can remove the piston seal.  It is corrosion in the groove behind the seal that causes most seizing.  I use a dental pick to clean the groove that the seal sits in.  You need to get into the corners of the groove and get it all out.  The crud is usually a white almost powdery substance that removes fairly easily.   If the seal isn't damaged it CAN be re-used, but a new seal is preferable.  Most times, the piston just needs a clean up with, say, 400 wet&dry and it will be fine, unless it's badly pitted.

[Smithy]

Thank you Carolyn that's really helpful. I think I will bleed all corners so I have a fresh start