Hi all,
Here's what I have going on. Back in December, as I was driving, a usually stiff brake pedal suddenly went to the floor. I topped off the fluid and the brakes worked okay, pedal was still soft though. I lived with the soft pedal until yesterday when the brakes failed altogether. As far as I can tell there is no fluid leaking anywhere. I can hear the brake booster trying to work by the audible hiss when I depress the pedal. The master cylinder is still full of fluid from the last time I topped it off. What am I missing? This thing has no brakes and by everything I've looked at, it should.

 

The brake master cylinder is a pump. I'd get a helper to pump the brake pedal as you attach a tube to a brake bleeder and try bleeding, see if the MC actually pumps.

Sounds like fluid was real low, and you got air in the brake hydraulic system. I'd get a helper to pump the brake pedal and try bleeding; if that restores brake operation, then you should carefully check the hydraulics for leaks: lines, calipers, wheel cylinders. But most often a master cylinder will leak at its rear seal, into the brake booster, so that might not show an external leak.

 

Thanks Cusser. I also googled the issue and found that the booster check valve may be bad allowing air to get into the lines. So I got one of those I'm going to install today and bleed the brakes as well. I'll post about the results later.