getting dangerous- replacement calipers needed.

[citronut]

i have always understood these calipers have a conical threaded shaft inside, and as the pads wear the piston is adjusted out further, and the cam on in the back is just to apply the hand brake,
this is why if you just lever the piston back you destroy the adjuster mechanism inside the caliper,

also if there were no thread involved you would not need to wind the piston back, you could just push it back in,

regards malcolm

[Geoffrey Gould]

Hi As above, there is a shaft with a very coarse thread almost like an 'achme' ( spelling?) which enters the piston and goes into a sort of 'one way clutch' which is the only way I can describe it. As the piston comes out of the caliper to take up wear/play in the pads then the shaft is gripped, the shaft is connected to the handbrake cam and this is what pushes the piston out. As the piston travels out being as it's 'locked' to the shaft that is the adjusting mechanism. The piston can not rotate as it's locked by the pip on the pad locating in the slots in the piston.
The only way to retract the piston, with out damaging the mechanism,is to rotate the piston to wind it up the thread. Not just rotate but push as well, this defeats the 'clutch mechanism'.
Sorry if that was a long winded way of an explanation but it really is a simple and clever mechanism but hard to try and picture with out an err picture. There is a little bit of slop built in to allow the piston to retract so the pads don't stay 'on' the discs when the brakes are released.
Incidentally the pistons rely on the rubber piston seals to retract the pistons. The pistons come out and distort/twist the seals and when the brakes are released then the seals go back the their proper shape and take the pistons with them. Tis only just a fraction.
If you really want to see how it works and have a caliper spare then just push the piston out with fluid and all will be exposed. You can just wind the piston back in, it's a long way but it does go.That is how you change the piston seals and dust cover.
Just needs somene with a caliper and camera and all will become clear, I hope.
Cheers.
Geoff.

[Vanny]

Just needs someone with a caliper and camera and all will become clear, I hope.

Hmmm, I might just have to do that. I've always just pushed the calipers back in with a set of Irwin Quick Grips;



The nearside caliper is one of the few original parts on my BX, and other than a bit of a re grease it has never been meddled with. I can assure you that the handbrake bit works just fine, as my parents rather steep drive can attest to (many a BX has been lost off that drive!).