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The other morning I got in the car and the brake pedal has no pressure at all. No warning at all. Fine the day before. Temp was around 35f. No brake fluid leaks anywhere and brake fluid reservoir is full. Thought bad master cylinder so I replaced it. Bench bled. Bled all 4 corners in the traditional method. Fixed. Next day same thing. No brakes. Just bled them again and had air in the lines. No fluid leaks anywhere. It is fixed again but I have a feeling they will be gone again in the morning. Any ideas? 95 bonne SSEi by the way.

hi, this is strange indeed!! I would get her off the ground and look and feel for a leak. You will see the fluid soak metal and any painted parts will bubble.

are you using the specified fluid for her? IIRC its dot 3.

Did you install a brand new master cylinder , a rebuilt mc or a junkyard mc?
If anything other than 100% brand new, I would suspect the replacement master cylinder is bad.
Rebuilt mc s aren't worth the money saved, as they're often bad.
How often?
When I worked as a tech, we did not install rebuilt masters--ever.
If you want to be sure there are no leaks, try this.
After you have the brakes bled, rinse off all connections with water.
This will remove any brake fluid residue.
If there is a leak at any fitting, it will be apparent after driving, since it will be wet with brake fluid.

So far so good it seems. I guess I had some air in there somewhere and it just didn't show up until a day later. I'll give it a few more days and see what happens. Rebuilt MC and DOT 3 fluid by the way.

No go. It happened again... Gotta be air getting in from somewhere. Maybe some bubbles were stuck in the abs whatchacallit. I'm bleeding them again and if it happens again I'll replace the MC with a new new unit. Then I'm just going to have to start blindly replacing things I guess if it happens again...

The only way air can get in the system is a leak. Which lines have air in them - all or specific ones? Just have to find the leak.