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Front brake service / set up 400/4 advice please

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Rozabikes Tim:
Went for it and bought piston and seal kit. Also silicone based fluid as suggeted. Hopefully get out to the garage at weekend. Told esssential to get rid of all  residual conventional fluid. Any tips on flushing? Thinking meths then air line?

K2-K6:
As I understand it generally with calipers the seal operates in a very particular way which you need to make sure it's able to do.

The groove in the caliper body has the face (back face looking into the bore) at 90 degrees to the the bore the piston travels in. The front face (the one nearest to you which you can't see) is at a small angle which is leaning outward towards you.
When the piston is at rest with no pressure on it the the seal sits square to both the piston and the back of its groove. When you use it, as the piston moves outward it drags the seal with it but the seal in the deepest part of the groove remains static. Because of the slope on the groove is in the direction of piston travel, the seal deforms in cross section to a parallelogram shape rather than a square section. Then when you let go of the brake lever it's the seal deformation that as it recovers back to its original square section that retracts the piston into the caliper. This is absolutely vital to the caliper functioning as it should.
If the piston fails to retract then the pad is always applied gently to the disc. It's this status that gently glazes the pad and makes it squeal, also reducing it's ability to generate friction and voila,  crap braking is the result.

The taper in that seal groove is where corrosion builds up as it has no additional seal to protect it. The seal becomes held static by the corrosion filling up its movement space so that when you use the brake the piston is forced straight past the seal without deforming it,  so no retraction takes place and it just drags all the time.
To make it all work properly,  that seal groove has to be scrupulously clean,  the seal needs to be of the right material/dimensions and you may want to assemble it with brake specific rubber grease.

RGP750:
K2-K6 Great info there , thanks for that.
Understanding the full function of a design really helps when restoring anything.

Trigger:
And as I have said before : treat it as a whole system, just changing or looking at one part will not help. You have to go through the lot if you want everything to work as it should.

Rozabikes Tim:
Fab info chaps.

Any specific checks on flushing system for silicone fluid change?

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