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Rear Brake Caliper Theory?


Guest smegheadsteve

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Guest smegheadsteve

Hi all.

Took my 2b for it's Mot today,to cut a long story short,recently fitted new rear brake disks and pads,got to test centre and there was bad burning smell.Turns out it's the offside rear caliper seized on.

He passed it,I said I said I will look at it!

Anyway,when I fitted new pads,this caliper piston was really hard to wind in,but I carried on regardless.

Now to my question!Even with a Haynes manual I can't get my head around how the caliper assembly works.If you have to wind the piston in,does it then wind out with hydraulic pressure,or does it push out straight?

So then,If I have to wind it back by hand,how does the caliper manage this on it's own to release the pressure off the disc?

I have eliminated the handbrake cable by pushing the spring out further but the disc still will not turn.

I would just like to understand a bit more before I take it all apart.

What could cause the piston to be really difficult to wind in,and will I be able to repair it,or is a new caliper needed?

Cheers guys.

Steve.

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Guest Tim Norman

They are stiff to turn are you turning it the right way? from memory with new pads the piston need to be almost flush with the calliper to get them to fit.

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In the caliper bore there is a mechanism that is held in by a circlip. It's relatively easy to get out but quite awkward to get back in! It is doable with patience though. The handbrake acts on this to push the whole mechanism up the the bore. The large spring pulls this mechanism back down the bore. This mechanism has a threaded bolt upon which you will find the piston. The hydraulic pressure is more than enough to push the piston up the bore while turning on the thread to act on the disc. As the pressure drops the mechanism relaxes just enough to drop the pressure off the pad.

On a good caliper it is relatively easy to wind the piston back in and out, if you can't then maybe there is some corrosion? Get an overhaul kit from Biggred and replace the inner and outer seals and clean both bore and piston and all should be well. If the handbrake action is free and easy with no leak at the handbrake pivot seal then you don't need to take that inner mechanism out. One of my handbrake mechanisms was very stiff, once I had the piston out I worked it back an forth several times and it freed off. The other one leaked from the pivot seal and had to be completely stripped.

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when I do rear brake discs unless you got the proper brake rewind tool use a g clamp and mow grips.while tightening the g clamp you need to turn the caliper piston clockwise I think until it is all the way flush with the caliper. to alter handbrake adjustment it is a case of turn it the other way until the caliper is just a nice snug fit over the disc with the pads in place. hth

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