Ever since I was a kid I've never been able to pass up the chance of taking something apart to see how it works. The only difference with age is I've gained the patience to put it back together again 
Anyway, one of my brake callipers needed attention the other day, so I thought I'd share what I found for all the other big kids out there...
It's a very simple design really - only four moving parts (or five if you include the oil!)
In the first picture you can see all the components - the two halves of the calliper body, the white ceramic pistons, square profile piston seals, bleed nipple, and bolt that holds one end of the calliper together. The other end of the calliper is held together by the banjo bolt, which is hollow and also delivers the oil from the hose.
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The close-up shows the recess the piston seal sits in (A), the oil channels drilled into the calliper body (B&C), and the threaded hole at the top of the picture where the hose attaches via the banjo fitting (D).
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This is how it works... When the lever is pressed, oil is forced down the hose and into the calliper through the banjo fitting. It passes through the small oil channels drilled into the calliper, to a recess behind each piston. You can see the opening of the channel from the banjo bolt (B) on the right of the close-up picture above. The opening on the left (C) goes to the bleed nipple to allow air to be bled out of the calliper. The other half of the calliper body only has the one channel to carry oil from the banjo fitting. The increasing oil pressure behind the piston forces it out, and it presses the brake pad onto the rotor - slowing you down. All basic hydraulics so far, but here's the clever bit - the rubber seal is a tight fit round the piston, but a loose fit in its recess in the calliper body. So, as the piston extends it drags the rubber seal with it, distorting its shape. When the lever is released, the 'squashed' rubber seal wants to spring back into shape, pulling the piston back in a millimetre or so with it (this is also the process that makes the callipers self adjusting as the pads and rotor wear - the piston will always be forced out until it touches the rotor, but will only spring back a fixed amount). Meanwhile, as the pistons recede, the metal spring between the pads forces them apart and away from the rotor.
Very clever, but deceptively simple really!
Dissembling and re-assembly of the whole unit was quite easy, so if only Shimano would do spare seal & 'O' ring kits I reckon they could last for ever!
Anyway, one of my brake callipers needed attention the other day, so I thought I'd share what I found for all the other big kids out there...
It's a very simple design really - only four moving parts (or five if you include the oil!)
In the first picture you can see all the components - the two halves of the calliper body, the white ceramic pistons, square profile piston seals, bleed nipple, and bolt that holds one end of the calliper together. The other end of the calliper is held together by the banjo bolt, which is hollow and also delivers the oil from the hose.
The close-up shows the recess the piston seal sits in (A), the oil channels drilled into the calliper body (B&C), and the threaded hole at the top of the picture where the hose attaches via the banjo fitting (D).
This is how it works... When the lever is pressed, oil is forced down the hose and into the calliper through the banjo fitting. It passes through the small oil channels drilled into the calliper, to a recess behind each piston. You can see the opening of the channel from the banjo bolt (B) on the right of the close-up picture above. The opening on the left (C) goes to the bleed nipple to allow air to be bled out of the calliper. The other half of the calliper body only has the one channel to carry oil from the banjo fitting. The increasing oil pressure behind the piston forces it out, and it presses the brake pad onto the rotor - slowing you down. All basic hydraulics so far, but here's the clever bit - the rubber seal is a tight fit round the piston, but a loose fit in its recess in the calliper body. So, as the piston extends it drags the rubber seal with it, distorting its shape. When the lever is released, the 'squashed' rubber seal wants to spring back into shape, pulling the piston back in a millimetre or so with it (this is also the process that makes the callipers self adjusting as the pads and rotor wear - the piston will always be forced out until it touches the rotor, but will only spring back a fixed amount). Meanwhile, as the pistons recede, the metal spring between the pads forces them apart and away from the rotor.
Very clever, but deceptively simple really!
Dissembling and re-assembly of the whole unit was quite easy, so if only Shimano would do spare seal & 'O' ring kits I reckon they could last for ever!
