Top tip for re-assembling pistons into brake calipers

[GPW]

(Any way to add this to the wiki?
http://www.e30zone.net/e30zonewiki/inde ... s#Calipers

So, I was making the pistons in my brake calipers move properly again by cleaning the sticky gunk out.. when I found a useful tip using compressed air. I used a telescopic duster attachment (like a car ariel with a trigger), but a foot pump would probably work fine too. My air duster gives a maximum pressure of a just few PSI, I wouldn't recommend using any more.

Piston removal:
Air can be used for this, but empty the fluid first and use blue paper to stop spray. Also use a 20mm bit of wood to stop the piston shooting across the room and keep your fingers out of harms way.

If it doesn't pop out then push it back in and it will tend to pop out fine next time you try. When the bellows have expanded enough carefully unhook it from the groove, use air to push it out and it'll make a 'pop' as it leaves the caliper. Use wood to stop it, not your fingers.

Cleaning:
I had cleaned out the crud in my calipers, soaked a rag in brake cleaner and cleaned the piston and bore to a nice shiny clean state.
Make sure the piston and bore are both smooth and clean.

Use silicone grease to lube it up in the caliper bore and piston surface, and also the rubber bellows inside and outside where it fits in the groove - we need that bit slippery.

Don't use ordinary grease, plumbers merchants sell handy pots of silicone grease, but wear blue nitrile gloves as it's tricky to wash off.

Re-assembly - the tricky bit!:
Sticking it all back together sounds easy - but I could not get the piston back in as the rubber bellows was in the way, it's smaller than the piston as it fits tightly into the piston groove. Also one's gloves and the piston are coated with silicone grease...

What I really needed was something inside the caliper, pushing the rubber outward to stretch over the piston, it took a while but eventually I twigged, a small amount of air pressure is the key.

Put some wood in to hold the piston just up to the bellows (quite a bit of force pushes the piston even just push-sealed the the rubber bellows). Applying a little air to the inlet hole and perhaps jiggling the piston slightly the air pushes the inflated end of the bellows right onto the piston. If the top goes on but not the bottom, move the piston up and the bottom will also pop into place.

Then just push the piston in gently and you'll feel it engage, pop the rubber edge into the slot and push the piston home. If it's not sliding in nicely use the air to move the piston in and out a couple of times and it'll seat properly. When done it should smoothly and easily slide and your brakes will now work far better.

[Topblag]

Good thinking with the air for refitting, it can take a while using your fingers and small blunt implements.