Hi all,
I’ve got a sort of theoretical question: how does pad/piston progression work with hydraulic brakes. As pads wear and the pistons reach out more and more, with the system closed, wouldnt that make the pressure of brake fluid fall? So does it? Does it even matter? It’s a sort of thing that hit me while riding and has been bugging me ever since.
Cheers
Most systems aren’t actually closed. The master is vented and there is a rubber diaphragm that prevents fluid leakage but allows air to fill in behind the master cylinder.
Simple answer: the self-adjusting mechanism is facilitated by a uni-directional check valve from the master cylinder to the “active” part of the brake system, keeping the fluid level in the line constant, independent of the wear conditions. As Scott mentioned, that back-fill is allowed for by the semi-permeable air port, keeping pressure constant in the master cylinder.
Longer answer: A hydraulic brake system can be broken down into Lever - Master Cylinder -/- Brake Line - Caliper Body - Piston. The slash represents a uni-directional check valve. When new, the entire system is full of fluid; master cylinder topped up, and line/caliper full. As the pads wear, when you pull the lever, more fluid flows into the line (the “active” part of the braking system) from the master cylinder. This extra fluid is prevented from returning to the master cylinder by a check valve. This process continued until you hit the design limit of wear for the brake system.
Two factors.
One, at the caliper end. The piston movement is typically caused by flex of the seals, not by them sliding in the bores. Only when they can’t flex enough (due to the pads wearing down) do they slide forwards to advance. This breakaway behaviour is dependent on the brake fluid properties, hence the right fluid is important. But this is what causes the demand for more fluid into the system while also allowing the pads to retract from the rotor correctly
Two, at the master cylinder. When the lever is fully released, there is an open port between the master cylinder and the reservoir. This allows extra fluid into the cylinder due to the piston advancement. This port is closed as soon as the lever is pulled. (The port als allows you to push fluid backwards when retracting the pistons.
If the check valve is unidirectional, what happens when you replace your pads and push the pistons back out to the “new” pad position? Does the check valve have a high pressure release built-in, to allow the excess fluid to go back into the reservoir at the lever?
IDK the technical details, but I can attest from experience that if one resets the pistons without opening the bleed port at the lever, one can burst the bladder that holds brake fluid in the lever. I’ve, uh, done this several times with my Shimano XT brakes, despite knowing I need to be careful. Fortunately, Shimano sells replacement bladders.
It’s not a check valve, it’s an open port. The port is only closed by pulling the lever slightly.
Man I love this forum. Thanks for quick and precise info!