Disc brakes

[holmslaw]

Recently replaced my front disc pads - no problem. What I cannot understand is, to replace the pads you have compress the brake piston. So in normal operation how does the piston move away from the pads after you have applied the brake. Totally baffled. Thanks in advance for what I'm sure is a simple explanation.

 

[JohnD]

All it has to do is release the pressure. The pad may still be in contact with the disk but if there is no pressure it will not give a braking effect.
In fact, the pad will usually relax away from the disk half a smidgin, partly by being pushed to the extent of any run-out on the disk, and partly with inertial forces as you drive about.

If the cylinder is very dirty, the piston may jam and not relax back, in which case the pad may rub on the disk and overheat until it wears away enough to give some clearance.

 

[johnwr]

Hello holmslaw, JohnD has got it mostly correct, the way the pads are retracted is as follows, when you apply the brakes the piston or pistons move outwards, if there is normal pad to disc clearance the caliper seal will tend to grip the piston and deform slightly, so when the brakes are released the deformed seal will pull back the piston so giving a slight running clearance between pad and disc. I am not sure what johnD's inertial forces are, can you enlighten me.
regards johnwr