I have a strange behavior on our '04 4R, the brake pedal goes to the floor on occasion, a pump or two and it then works fine. We have found that it will only happen if we have the car in reverse, then to drive it will go to the floor the first time we press the brake pedal. The brakes work fine in reverse and forward after that first press after being in reverse.

The brake fluid level is at full, there are no leaks, and no other known brake problems.

Any idea what would cause this? I am totally stumped.

Edit 7/15/15: After further testing, I've found that it has nothing to do with reverse gear. I let the vehicle roll backwards in neutral on a short decline, come to a stop without brakes, and then put in drive and moved forward. The brake pedal went nearly to the floor. So just rolling backwards triggers the behavior.

I have an appt with our local repair shop next week.

I'm guessing an issue with the master brake cylinder?

Had it happen on my Lexus LS430 a while back.

Sounds like the master cylinder to me as well. I Would plan on replacing it before it fails when you really need it.

It was only front brake pads.