- Fri Jan 26, 2018 2:16 am
#73724
It's perhaps worth taking a brief moment to think about how a hydraulic disc brake works?
So you pull the lever which causes fluid to be drawn out of the reservoir by the master cylinder piston, this causes an increase in pressure in the hose then the calliper. this causes the piston to be pushed out and the outer pads to be pushed against the disc. This in turn causes the calliper to move slightly on the sliding pins causing the inner pads to be pushed against the disc.
That's the simple bit. The odd bit is why, when released, don't the pads simply remain resting against the disc? In a perfect, friction-free system, they would. You'd release the lever which would release the pressure on the pistons and they would stay where they were left. So what makes them move away from the pads? Because your problem is here somewhere. A little will be due to runout on the disc forcing the calliper along the sliding pins and the pistons back in but increasing disc runout isn't your solution! Most of it is mechanical stiction. The pistons don't actually move in the seals, the seals are still gripping the piston and deforming slightly as they are pumped out. Same thing with the rubber boot on the sliding pins.
So the piston actually gets pushed out a little way, taking the seal with it, then slips back in again as the rubber seal returns to its normal shape. As the pads wear, every so often the piston will "jump" out a little and actually move in relation to the seal. That's why you don't need to keep adjusting hydraulic brakes. (and why, as anyone who owned a CB100n will tell you, mechanically operated discs are a stupid idea)
There's also the question of residual pressure in the line. If the master cylinder piston were to return to a "neutral" position after release, you'd only ever move the same volume of fluid back and forth, the pistons could never move further out than they do in one stroke. So they're desinged to move more fluid out than they allow to come back when released, each new stroke picks up a little extra fluid. Obviously if this carried on, you'd just keep pushing the pistons further and further out. To prevent this, when the lever returns to the "fully out" position, a tiny return pnhole is uncovered which allows the pressure in the brake line to equalise with the pressure in the master cylinder (ie zero, or maybe a slight vacuum). So that little extra push back by the seals flexing causes a "squirt" of fluid to pop back into the reservoir when the lever is fully released. You should be able to see this. It's sometimes of high enough pressure to throw brake fluid up in the air and start taking the paint off your tank (wipe immediately with a damp cloth)!
So the main things that allow your pistons to return away from the disc are the stiction between the seals and piston. Stiction between the boot and pins and the return circuit in the master cylinder. Anything that compromises the action between the seal and the piston will cause sticking. In my experience, this is much more likely to be corrosion between the calliper and seal causing the seal to deform than dirt between the piston and seal but also, if the piston has been very dirty, it can have damaged, deformed or twisted the seal. A twisted seal was the cause of a sticking piston recently on the pretech 6-pots on my VFR. Also consider the return circuit. It can be blocked with a tiny piece of dirt or crystallised brake fluid. It can also be that the lever is not fully returning to "off". I once had a bike with hand guards, the lever was fouling them and it resulted in the brake "pumping itself up".
I've personally never seen a brake hose go bad enough to stop the brakes actually working. Right up to the point where they catastrophically fail altogether.
