Brake Failure/Locked up

Driving yesterday, (slowly down bumpy gravel road). My 2005 Prius locked up, the red BRAKE and the yellow (ABS) indicator lights came on. The car wouldn't go forward or back. I tried setting the emergency brake then releasing it - nothing. Still couldn't go forward or backward. I shut it off, turned it back on, nothing, tried setting the emergency brake then releasing it - I finally was able to drive, but the indicator lights were still on and when I tried to brake, there was nothing, nothing, nothing, then BAM the brakes grabbed and locked up. (This happened several times in a few blocks as I was cautiously trying to test the car out.)

I parked got out my owners manual which said "STOP, contact Toyota Dealer" so I had my car towed (first time ever for her to be towed) to my mechanic (he's out hunting, so someone else at the shop looked at it.)

The shop called and said it was showing a code of 1247 and said that it was a "malfunction of the brake pedal".

They said they cleared the code and the lights went out. Their mechanic is going to take it for a test drive trying several 'braking scenarios'. (they better not wreck my car).

Anyone hear of this happening before?

 

Towing was free (well... not exactly free: Allstate Road Service). Shop just called, rear brakes were "completely out of adjustment", master cylinder was "overfull".

They are going to clean and adjust the brakes, take the fluid down to the proper level. $151.48

They said that if my oil change place had not been refilling the Master Cylinder - my car would have 'told' me that my brakes needed attention (that's how the system is designed to work). I plan on putting a sticker under the hood that says DON'T FILL.