Just what Dave said.Meritor Auto slacks have a pawl pin on the side of them thats spring loaded Its about the size of a pennie and When you are under the tk looking at the side of the slack adj faceing you it is right on the side of it..Get a small screw driver gently pry the head of it outwards you are releaseing the lock doing this. is.Now you need to get a small hex 1/4 plug socket.On top of the slack is the manual adjuster.While holding the lock pawl out rotate the adj counter clock wise till the End of the S-cam stops turning inside the slack and the body of the slack itself just starts to move.At this point you have turned the S-cam back to its neutral position and its cupping the shoe rollers inside the drum.Now looking inside the drum you should be able to plainly see the shoes are fully retracted and The brake is fully backed off.If it is a Bendix auto slack adjuster it will have a 7/16 bolthead halfway down the rear of it.This is the manual adj.It has a auto lock inside the slack.To back it off you need to turn it counterclockwise.You will hear the lock pawl skipping inside of it while you do this and feel alot of resistance.This is how it is done and will not hurt anything.After you get the Brake job done and drums and wheels mounted.If the tk has maxibrakes make sure it has 90 psi of air, chock your wheels release the parking brake and adj the slack adj.Or if you have previously caged the springs in the Maxican you are OK.Crank the adj down till the lining contacts the drum.Then back it off 1/2 turn.Any questions on anything please ask I work on rigs for a living and can help you out on any problems or questions .Not to be smart but Spring brakecans are dangerous and can kill you so if you have any doubts ask someone first before doing something you may be unsure of.Thanks
I had to pull the brake chamber to let some more pressure off the slack adjuster.
It has auto slack adjusters. there is no backing up to them.