Slack adjuster adjustment is required to keep the brakes in tip top condition. Before you begin the slack adjuster adjustment, it is important that you block the tires to prevent the trailer from moving. You can do this by raising the truck / trailer by using a jack stand during the adjustment.
Make sure to have the brake system at full pressure; lower the emergency brake and push in the air valve. Now listen for any air leaks as they lower the braking ability of the truck / trailer. Now, use a piece of chalk and make the push rod where it leaves the air chamber. Then use a screwdriver for leverage and pull the rod out of the air chamber and measure chalk line to the air chamber.
Now, locate the manual slack adjuster bolt which connects the s-cam to the air chamber on the axle housing with the push rod. Then turn the slack adjuster using an appropriate sized wrench. Make sure that the s-cam is moving in the right direction.
You should keep on tightening it until you feel the resistance. Now, reverse the slack adjuster one-quarter to one-half to put a little distance between the drum and linings. Apply the brakes with normal pressure and see that you have no more than one-tenth of an inch between brake linings and drum.