Stan944

Here is my question:
There is an air bubble above the fluid in the brake fluid reservoir. I would think that depressing the brake pedal would compress this air before engaging the brake. This would cause the brakes feeling spongy at the beginning. But it is not so! Following this thought of compressing the air bubble, if the reservoir cap was missing, the fluid would leak out during braking (and I don't think it would happen in reality).
Some explanation please?

Dave951M

That's because there is a "check valve" type of arrangement in the M/C. When you depress the pedal, the piston inside of the cylinder moves, covering the opening that allows fluid into the M/C, blocking any back pressure from pressurizing the resevior. It's a general explanation but conveys the idea. You can leave the cap off of the resevior and have no problems as far as pressure goes. However, brake fluid is hygroscopic, meaning it absorbs moisture from the air. The moisture content can lead to internal corrosion and failure of various brake components and brake fade under heavy braking. It also keeps the fluid from sloshing out and keeping other contaminants from falling in. Yes, you do need the cap and yes, you should have your brake fluid changed about once per year, more if you track or drive the car hard.

Stan944

Thanks, it makes sense now.