2006 brake assist when not wanted,,,

You are talking about the B mode shift selection? You are saying its jumping into the B mode by itself and you see B illuminated as a gear selection on the dash? Don't remember any posts about that here on PC.
Maybe the gear selector is faulty.

Is the car showing any warning lights on the dash?

How long have you owned this car?

 

I figured you had not owned the car long as its immediately noticeable when in B mode.The car is very sluggish in B mode.
Cannot remember any posts about the shifter doing that. Seen a few posts about the gear shifter jumping out of gear but none about jumping into B.
I bet you just may be knocking it into B by mistake.

The tire pressure sensor's will illuminate if the tires go lower than 25 lbs I think. Can't tell by looking at the tires you must use a gauge. Put the tires at 40 front and 38 rear to start. Will really help the mileage.
If the tire looses air again it either has a nail or the tire needs to be replace as the bead is getting loose on it.

And if the car has Goodyear Integrity's on it replace them asap as thats the worst tire ever made. Your car will drive 100% better with any other tire made besides a GY Integrity.

 

Honestly, if you've accidentally done it once then by far the most likely thing is that you just bumped it the second time as well but didn't notice. It really doesn't take much of a bump to throw it from "D" to "B" (or visa versa). It's not like a regular "mechanical" linkage, it's basically just switching a software mode and is something that's meant to be simple and smooth to do when driving in mountains. It literally only needs a fairly simple flick from side to side to toggle between one mode and the other.

No, apart from perhaps a small reduction in fuel economy there is no ill effect on the Prius due to using "B" mode. The differences in mechanical "wear and tear" are pretty much negligible, however using "B" is just a little less efficient. So the majority of the time it's best to just use "D" mode, except for a few specific circumstances like very large descents, where "B" mode may actually reduce brake and battery wear and tear very slightly.

 

I wouldn't bother trying to tape off the shifter or anything, you'll get used to it soon enough. And anyway it's not going to hurt anything if it occasionally gets bumped to "B".

 

The one thing that will bone you good on this car is the little 12 volt battery in the trunk. Its called the aux battery and the "boot" battery as its basically just closing relays when you hit the start button. That little 12 volt battery does not like to go dead and after it has sat dead for a while it will be troublesome when re-charged. It will not hold a charge very well.
The very worst thing you can do to this car is come at it with jumper cables. Hundreds of posts about people trying to jump start this car because that little 12 went dead and causing alot of electrical damage to the tune of $ XXXX.
Do all you can to avoid killing that 12. This is a great little car but do not futz around with the 12.

Since you bought it at a dealer I can almost assure you that battery has gone dead maybe more than once which means its a ticking time bomb.

Lets test it. Here's how.
If you have a dc volt meter thats very handy. Open the hood and remove the black fuse cover over the drivers side front wheel area. Push in the top edge of the bottom cover where it meets the top cover. That will release the cover. You'll see a red plastic tab. That red tab is covering the 12 volt jump point. Flip that tab up and you'll see a little L bracket. You attach the car jumper lead there to that tab if you have to jump start. You can use one of the nuts that holds the strut on right above the fuse box as the ground point. I want you to measure the battery dc voltage on that red plastic tab to ground.

If you don't have access to a 12 volt meter you can do it this way. The meter is more accurate:
Weird stuff happening? MPGs dropping? Test The Battery | PriusChat

If using a meter: With parking brake set and your foot NOT holding the brake down push the start button once...wait a second..then push it again. Thats twice. Now the car is in accessory. That will pull max load on the 12 volt battery.
Then go measure that 12 volt tab to ground. Anything below 11.7 volts and it will need to be replaced. I bet you will measure 11.4.
When the battery is real low it will throw an instantaneous red exclamation point on the dash as it boots up. Will only show for a second then go away and will not set a DTC. That's the only warning you will get. Next stop after that is a car that will not boot up at the most inopportune time.
Many different battery's to choose from. Hundreds of posts about this.

Report that voltage back here.