I have a dewalt drill model #dcd970 where the brake is not working. What could be the problem? And how can I fix this?

If you can to take apart the drills, put the battery directly to the motor. If it is turning the , probably the switch is faulty. If it is a controlable speed drill, check the FET transistor. I recently replaced the FET in a Ryobi drill. Cost $5 and workin glik ebrand new.

Remove left have screw inside check, install large allen wrench in jaw of chuck, using hammer hit CCW to loosen chuck.

I don't think the motor is serviceable that way. At
DeWALT DC970 Parts List and Diagram eReplacementParts com you can find the motor and pinion for $35 plus shipping and then get the entire drill working again. Had the same problem with one of mine and it was an easy fix.

I'm assuming you have a DCD970 as DeWalt doesn't have a DC970 and the DW970 is an impact driver. The chuck on your drill is a new design that doesn't have a lock screw inside the chuck, they are put on very tight with lock-tite to hold them on. They are made to be taken off with a special chuck holder and vice jig that only service centers have. The allen wrench method used on older chucks won't work as the chuck will just loosen when you hit the allen wrench. I have on occasion removed one by doing the following: Turn the chuck fully open then secure it in a vice with the drill straight up. Holding the drill body and using the handle for leverage, turn the drill counterclockwise, as you are looking down on it, with steady, strong pressure. If the chuck breaks free it will spin off easily. If you are at all afraid you may break the handle you may have to take or send it to a service center to have it removed.

Your cluck is worn and needs to be replaced, below is a link to a video of how to remove the chuck.

Link:

The chuck on your drill was made to be taken off with a special jig and holder that cost $175+. Depending on how tight and how much lok-tite the factory used some chucks can be removed without the jig. You need a hex shaft at least four inches long and 3/8" to 1/2" wide. Lock the hex-shaft tightly in the chuck, then put the shaft in a vice with the drill straight up and down. Using very firm and steady pressure, turn the drill using the handle for leverage until the chuck breaks free. Don't bounce the pressure or use an impact wrench as this is very ******* the transmission. When you put the new chuck on, use a few drops of lok-tite and put it on as tight as you can without breaking the case. Let the lok-tite set up for at least one day.

check the inside of the chuck at the bottom(open the chuck to max). There should be small screw there with either a hex or a Phillips head. It is a left hand screw that locks the chuck in place and will not let it unscrew when you are removing parts.. If it is not there then you will have to get one from an old drill that is junk unless you can get one from an electrical appliance shop the repairs drills.

key less chucks are not the best .. You can get bits w/ squared sides .. or use channel locks to grab chuck & tighten better,,

first make sure brushes are not worn out and if not then replace the switch. the brake is contained in it to throw a short thru the motor when trigger released to stop drill. You can find a breakdown for this tool at Dewaltservicenet.com

yes this is normal the spark keeps motor from stalling out by taking away the friction spark by making own spark
the clunk is the drill brake so you dont brake your hand when screw goes in all the way unlike old drills that kept going even when your hand was attached to it