No, that’s for cars.

Many cars don’t have a sealed system like MTBs. Because they are never turned upside down, they just have a vent in the cap. That’s the same as leaving fluid in an open container.

Brake systems are highly pressurized when you clamp on the brakes. The fluid on the inside is at much greater pressure than the air on the outside. If moisture could pass through the joints and seals to get into the system when there is no pressure differential, fluid would squirt out when you applied the brakes. This doesn’t happen because the system is sealed.

If moisture could pass through the joints and seals to get into the system when there is no pressure differential, fluid would squirt out when you applied the brakes. This doesn’t happen because the system is sealed.

There are microscopic holes in all the seals. You have to remember that DOT fluid is hygroscopic and actually attracts water.

Often a seal only seals properly when it’s under pressure so it’s entirely possible that the brakes won’t leak but can still absorb moisture.

According to your logic there is no need to ever change brake fluid. Do you ever change your brake fluid? Why?

My very first Hope brake had an adjuster on the top of the reservoir. Even when they were brand new they would lock on if left in the sun and you had to back off the adjuster to be able to ride the bike again.

C2s, the C being for Closed. (i believe)

In a closed system, the reservoir feeds the hose further down the valve, so when you pull on the lever, the reservoir itself is also under pressure. There is a plunger under the adjuster that pushes the fluid up and down exactly as the plunger on the lever does.

On a modern Open system, (i think) the oil reservoir port is positioned so that as you pull the lever, the reservoir is seperated from the pressurised brake system, and is never under pressure itself. As you release the brake the port opens, and the fluid can equalise its pressure/volume with the reservoir, which is open to atmospheric pressuire (save for a diapram that just keeps the air our the fluid and the fluid in the reservoir) to compensate for pad wear and also heating.

[edit]

Brake systems are highly pressurized when you clamp on the brakes. The fluid on the inside is at much greater pressure than the air on the outside. If moisture could pass through the joints and seals to get into the system when there is no pressure differential, fluid would squirt out when you applied the brakes. This doesn’t happen because the system is sealed.

only down stream of the lever. the reservoir doesnt pressurise, and is effectively open to atmosphere. The diaphram only keeps the fluid in the right place, but wont stop moisture getting in the fluid.

The diaphram only keeps the fluid in the right place, but wont stop moisture getting in the fluid.

It seals the fluid from the atmosphere. Moisture can’t get in unless the diaphram is damaged.

only down stream of the lever. the reservoir doesnt pressurise, and is effectively open to atmosphere. The diaphram only keeps the fluid in the right place, but wont stop moisture getting in the fluid.

That makes a lot of sense actually.

I’ve found that if the fluid has expanded so that the reservoir is effectively overfilled and I try to push the pads back to get the bleed block in the fluid is actually pushed out past the seal.

At the time I remember wondering how it was able to withstand the pressure of the braking system so your explanation clears that up.