General Refitting Front Caliper Piston

[Paul Simon]

First of all, a question. Should the piston be able to release itself from the caliper when under pressure from a pressure bleeding kit, or does it indicate a faulty piston seal?


I was in the process of bleeding the brakes, and took the front wheel off, and noticed that the disc was very stiff to turn, Without disconnecting the pressure bleeder, I undid the caliper bottom bolt, and removed the pads to clean them. A few minutes later, the piston popped out and dumped fluid on the floor (I immediately released the pressure and clamped the hydraulic caliper hose, and refilled the reservoir, which hadn't gone too disastrously low luckily). Luckily, the pads were out of the way.


If there's nothing wrong with the piston seal (still in the caliper), how do I reinstate the piston? Does the seal need to be removed first?

 

[portland_bill]

Pressure behind the piston will expel it, so this is not the problem. At least it is not seized.

To replace it, needs cleanliness and care.

Piston seal should fit in a groove in the caliper, so remains in one place. Make sure caliper is clean inside, use clean brake fluid or meths. (Meths needs wiping dry with a clean cloth or paper towel, but ensure no residue of wipe is left inside.
With piston clean, a little brake fluid on it and the seal will help, then ease it in carefully, making sure you get it inside the seal. Seal must not distort or come out of its groove. As the hose is now clamped off, remove the caliper and do this on the bench, or kitchen table.

Once the piston is back in, push it as far in as it will go, this will expel most of the air. Reconnect it, open bleed screw and let the fluid flow. Close bleed screw. Get an assistant to push pedal very gently to expel piston until about two-thirds out. Open bleed screw and push piston in by hand. This should expel all the air, making final bleeding quicker and simpler.

For final bleeding, replace the pads, as you know what happens if you don't.

Take care, and good luck.

Once all back together, check it regularly for a few days/weeks to make sure it is not leaking.

 

[Paul Simon]

Thank you very much Bill. That's an excellent reply.

 

[Paul Simon]

Managed to get the piston back in today. At first, I thought I'd never get it back, but it seems to be a case of keeping it square and persevering.

Copper greased the pad locator springs, and the discs now seem to move more freely with the pads in.

Many thanks again to Bill.