Makita Percussion Drill HP2051F keyless chuck release problem

watch this video and it will explain
Drill Driver 101 How to Use Keyless Chuck

The Makita BTD140 is an impact driver that has a hex-drive anvil and bit holder assembly, it does not have a keyless chuck accessory. There are some aftermarket chucks with hex shanks on them to put into impact drivers but I've seen very few that hold up.

Some times the selector on top get gummed up with dust, it just needs cleaning. You were right to put it into 1st gear to remove the chuck with a big allen key, you get more resistance in the gears.

Most drills with the keyless chuck work this way. Put the drill into reverse. Grasp the chuck with your left hand, and then pull the switch slowly. This should release the chuck. If your drill has no reverse, then grasp the chuck again with your left hand and hold the drill with your right hand. Turn the chuck with your left hand quickly, this should release the drill as well.

Dont know about a Makita, but my Craftsman drill , if you open the chuck all the way and look inside the hole where the bit goes, there;s a screw there that you can use to remove the chuck. amybe yours is the same way.

Go to this LINK and find the part/diagram that can help you.

Try two channel lock pliers.
Hold the bit close the the base with one.
Take the other (cloth comes in handy here to prevent marring) and turn the chuck counter clockwise to release.

Alternative (little WD40 on the chuck)
One channel lock plier hold the chuck and put the drill in reverse.
The WD40 should allow the chuck to release easier.

Good luck!

keyless chuck. grab the ring immediately behind the chuck and the tampered chuck piece with a padded channel lock plier. turn the chuck part clockwise to release.

There should be a Screw in side the end of the chuck were u put the drill bit in u will have to undo it back wards its got a left hand thread

First open the chuck jaws and remove the LH chuck retaining screw. After that find a bent handle allen wrench about the size of the chuck. Put the shorter end in the chuck and clamp down on it. Put chuck on a block of wood or workbench with the long end at about a 30 degree angle so you can hit it counterclockwise with a hammer. It might take a couple hits but it will come off.