Braking causes me to lose 1st gear

I have a 2014 2wd 4cyl automatic tacoma. In the last few weeks, when I am going down a steeper grade in first gear . . . if I apply the brake, the truck comes out of first gear (goes to neutral). What the hell is going on? When I release the brake, the truck goes back into 1st gear.

 

Have you checked your transmission fluid level? Sounds like it may be low.

 

Assume selector is in D?

Same if you manually shift?

 

When you are going down a steep grade , any transmission fluid moves to the front of the transmission pan away from the filter inlet . As others have mentioned , You sure your trans fluid level is up to snuff ?

You have the 4 cylinder . Be happy you have a transmission dipstick
Engine must be running to check for proper transmission fluid level