Emptying rear brake reservoir but no leak

And pressure maintained. Hanged a 30lbs dumbbell on the pedal, measured the placement. It has been 20hrs plus, no change in placement.

Hmmmm, sounds fixed. Hard to fix something that's not showing broken eh.

Gregger said:

Actually, for practical purposes they are sealed by a rubber diaphram which sits between the fluid and the cover. As the brake pads wear the fluid in the master cylinder reservoir is used, displacing the outward movement of the caliper pistons as they compensate for the pad wear. The diaphram deforms preventing air from entering the reservoir. Air can get in between the diaphram and the master cylinder cap but not under it unless it's torn or installed improperly.

Since brake fluid is hygroscopic, it actively absorbs moisture. I try to purchase only what I need to bleed the system. Any extra is thrown out if I'm going to store it for any length of time. Apparently this stuff can absorb moisture through the smallest openings.

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Actually, that's wrong. The diaphragm allows pressure to relieve when the brake fluid gets hot and expands. Then allows air in to compensate, trying to minimize water infiltration and brake fluid spillage. If you take it apart and inspect things you will find a small groove in the machined surfaces. The diaphragm itself is perforated to allow pressure in and out. Otherwise something would blow out or collapse under vacuum.

stingray said:

Actually, that's wrong. The diaphragm allows pressure to relieve when the brake fluid gets hot and expands. Then allows air in to compensate, trying to minimize water infiltration and brake fluid spillage. If you take it apart and inspect things you will find a small groove in the machined surfaces. The diaphragm itself is perforated to allow pressure in and out. Otherwise something would blow out or collapse under vacuum.

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?????

I've taken many master cylinders apart over the years and the diaphram was never perforated. It is designed to move up and down with the brake fluid preventing air and moisture from entering the fluid reservoir. Now as the fluid level goes down a vacuum is created below the diaphram. Atmospheric pressure above forces the diaphram down with the fluid. It can also go up with expansion because the master cylinder cover is vented to atmosphere. So the only air entering is what is above the diaphram. You don't want air getting into the brake fluid reservoir mixing with the fluid.

This automotive master cylinder reservoir cover shows how the diaphram moves/distorts to displace the lowered fluid level. The diaphram rubber would need to be reset to it's flattened position after filling the reservoir. There are no holes in the diaphram...