How often are you checking and adjusting trailer air brakes? Air brakes on MAC trailers should be regularly inspected. Poor brake adjustment is the most common cause of brake failure. When slack reaches 1-inch, just 1/2-inch more than recommended slack, your trailer brakes MUST be adjusted. It is best to leave 2-inches of reserve chamber stroke to ensure safe brake operation.
Brake Slack Overestimation Can Add Up to Trouble Fast
It can be tempting to skip a brake check when your brakes seem fine under light use. But with moderate to heavy braking, you may find yourself in trouble. With 80 psi of braking application, 1-inch of slack strokes 1 ¾-inches, leaving just 3/4-inch reserve chamber stroke. When hot, cast-iron brake drums expand, this can cause the pushrod to stroke a further 1/2-inch, reducing reserve chamber stroke to just 1/4-inch. As the lining of your brakes wears, that 1/4-inch reserve rapidly disappears, causing the chamber to bottom out and leading to brake failure. The takeaway: Don’t estimate – check the slack!
Brake Checks Are Required by Law
Federal regulations require brake inspections as part of your fleet trailer maintenance schedule. This includes a daily pre-trip inspection of manual and automatic slack adjusters. You’re also required to check brakes before driving down steep inclines. This is so important, “Steep Incline – Stop Here and Check Brakes” reminders are typically indicated on nearby road signs.
How to Check & Adjust Air Brakes on MAC Trailers
Manual Adjusters
- Pull out the pushrod, limiting its use with your hands or a pry bar. Measure the slack. Brakes with over 1-inch MUST be adjusted.
- Turn the adjusting bolt until you feel resistance. NOTE: The bolt may turn clockwise or counterclockwise! The bolt should be turned in the same direction as it does when you apply brakes. Outward movement indicates the wrong direction.
- Back off to 1/4-1/2 turn, then recheck push rod travel.
Automatic Adjusters
- If you have automatic adjusters, you must still check slack before trips and at brake checks. Why? Self-adjusting slack adjusters can fail or be improperly installed.
- Apply 80-90 psi. If you don’t have an application pressure gauge, set the pump reservoir to 90-100 psi and apply the brakes.
- Check the stroke, making adjustments based on your chamber type:
- 20 & 24 – less than 1 ¾-inches stroke
- 24 LS, 30, & 30 LS – less than 2-inches stroke
- Any measurements meeting or exceeding these specs require emergency adjustment by you or a mobile trailer repair service and rapid attention at your local tractor-trailer repair shop.
Don’t neglect trailer brake repair and maintenance needs. Ainsworth offers fast service for MAC trailers. Our trailer repair and maintenance express lanes get you back on the road in minutes to hours, with no long turnaround times. We even offer fast FHWA inspection on-site, helping you keep your tractor-trailer safe and up-to-date. Don’t cut your maintenance needs down to the wire. Ensure safe travels with the help of Ainsworth Trailer Repair today.
