Lazy ap caliper pistons

Hi,

Took off the right caliper to investigate pulling to the left when breaking. Inbound break pad was hard to remove, the caliper really needs cleaning out of baked break dust and grime, pads not sliding well. I'll clean it out and grind the edges of the pads a little and grease them, should help with that.

However...

I noticed only one of the pistons was out beyond the face of the caliper (sticking out at all). One of the outbound ones.
Put a bit off wood in the middle and pumped brakes.
Only one piston came out. The one that was slightly out.
Clamped that piston in with a g-clamp and left it clamped up. Pumped the breaks more.
Other outbound piston came out with one of the inbound ones having moved a bit.
wound that one back in, pumped brakes and it came out easier. Clamped that one in and kept it clamped.
Three pistons clamped in and a bit of pumping later the last inbound piston moved.
Removed all clamps, wood in the middle and made some poor sod pump the breaks while I watched caliper.

All pistons now come out but at different rates. The two outbound first then the two inbound move with one being faster than the other.

The dust boots look fine and I can see no corrosion or pitting on the exposed end of the pistons if I lift the boot rubber. No brake fluid leaking. Dustboots look fine.



Is this normal, do they just need cleaning or lubing?

Checked price of rebuild kits and pistons, to make these they must have to hunt down and kill Unicorns!!

Cheers,

Iain.

Sorry, double post on photos.....grrrrrrrrr.

After a lot of searching on the internet, it seems that in multi piston calipers, the pistons don't come out in unison or at the same rate. This info was from bike forums but I guess all calipers are the same.

They may have been sticky from lack of use as the car has been SORN for a year. I might try getting some red brake grease and slapping on the dust seals and around piston ends but I always thought just brake fluid was enough.

Reluctent to pull out the pistons as I assume the seals would need replacing then and it's not the cost but I read somewhere that the square-cut inner seal is ok but the outer dust seal is a absolute pig to fit. Anybody done this?

Seems it is cheaper to send them off to get refurbished than buying the parts and doing yourself which is a bit mad.

Well I can only refer to the only four pot caliper I have experience on , that being the London Taxi ( Black cab ) . They had a tendency to corrosion on the pistons and if you were careful and the corrosion not too crusty , you could clean the pistons and free them off . The outer 'Wiper' seal had a flimsy metal retainer that needed to be pressed into place when overhauling the calipers and yes they were an absolute pain to fit as they would kink and bend out of shape if care wasn't taken . The Wiper seal and the inner seal over time become dry and obviously contaminated with crud so it will help if you spray a bit of WD on the piston and work them in and out as best you can . It goes without saying that being a safety critical item , they need to be equal in their movement as brake imbalance / pulling will come into play as I'm sure you are aware.

Hi,

Sorry for long time to reply but wanted to test the car on a quite road on Sunday after finished playing with the breaks.

Car now travels straight with the breaks on and off and a lot better breaking overall.

Yeah, I thought that was how breaks should be, all pads grabbing the disc at the same time but thinking about it, even in a single piston sliding (floating) caliper, the pad against the piston must contact the disc first then the other side is pulled on.

Cleaned all the baked brake dust and crap out of calipers
Degreased everywhere with bilt hamber degreaser and scrubbed wear plates with a toothbrush sized wire brush followed by emery paper until clean and smooth.
Checked rubber boots on pistons for damage and smeared with red rubber gease.
Ground down the edges of pad back plates until they fitted back in caliper snugly but free to move.
Small amount of copper grease on sides of and back of pads (I know, not ideal but it's all I had).
Bleed front brakes and found one blocked nipple (never had that before). Cleared nipple by unscrewing and using a bit of wire to push through holes.

Much better braking and hopefully it will be ok for the MOT now.

Biggest problem with these calipers I think is the wear plates lifting out slightly due to corrosion/crap underneath. Grinding back the sides of the backing plate of the pads and lubing once a year is probably a good idea. On one of my calipers the pads were really jammed against the sides (the wear plates) and I don't think they were all moving.

Iain.