Brake fluid -where did it go?
 

The other day, all of a sudden, my brake pedal started slowly going down, as if it had air in the lines. I was having to pump the brakes just to get them to work. By the time I got home, the brake light had come on and the warning for low brake fluid. I opened up the reservoir and it was nearly empty. I filled it back up then drove it up on ramps so I could crawl under and see if I could spot a leak. I can find no leak. The lines all look fine, with no rust and the wheel cylinders are all dry. Where is this fluid going?

Nowhere. You have a bad master cylinder, just not building pressure.

The brake fluid is most likely leaking into the booster from the master cylinder and being then sucked into the engine.

Remove the plastic vacuum pipe from the booster and see if there is brake fluid in the pipe.

Didn't even think of that possibility, and I've seen it before now you've reminded me.

That's a reasonable possibility. I've seen master cylinders fail, and fluid gets a little low as it leaks past the seals, but this was virtually empty. I'll check that vacuum hose.

I found the leak! I jacked up the rear end this morning and took a really close look. I saw what looked to be a bit of oil on the righr rear shock, so I got in and stomped several times on the brake making it go to the floor. Then, when I went to look again it was actually dripping down thru the spring on the shock. Apparently a line up in there has burst. There was a bit of rust up in there so that must be the culprit. Good thing I didn't order the master cylinder.

That sounds more likely and less expensive! Got lucky!