Last week I got into my 2000 LS after not driving it for a few days and when I put it into reverse it went flying backwards with my foot on the brakes as hard as I could. A solid stand of hedges stopped me before I could turn it off or engage the emergency brake. I started it back up, pumped the brakes and all was fine. I replayed it in my mind and thought I must be crazy.

Then today, again several days after not driving it, it did it again, only this time time not quite as bad and I recovered it more quickly by pumping the brakes.

Brake fluid level in reservoir is about an inch from the top. I’ll take it in to the Lexus dealer here in Orange Park (FL) but wondered if anyone had experienced anything like this before? I’ve only had this car a few months. Thank you.

 

Air in lines that needs to be bled out, leak somewhere in system, or failing master cylinder, I think would be main possibilities. Obviously you are doing the right thing taking it to a professional for diagnosis

 

‘Thank you. I appreciate it.

 

sounds like a failing master cylinder! if you hold the pedal does it slowly sink?

 

Yep, just tested it; kind of squishy and then ever so slowly goes towards the floor. I’m going to get it in as fast as I’m able. Thank you very much.

 

Pretty sure that's the master cylinder seals. If it were air in the system, the problem would be consistent, and the brakes would feel spongy at all times.
When you press the brake pedal, and hold, the seals can't hold the pressure, the brake fluid escapes around the bad seals, and the pedal sinks in slowly as a result.
If you want to save money, take it to any Toyota mechanic and ask them to inspect your brakes. Or if you are confident, just buy a new brake cylinder and ask the mechanic to replace it. Even if you are not a DIY type of person, it is best to know how things work so you cannot be taken advantage of.
Currently rockauto has one remanufactured brake cylinder for 2000 LS400. You could buy it and replace it at any decent (Toyota) shop as this is not a terribly complex job, nor does it have anything Lexus-specific that I can think of.