What’s an Eighth of an Inch Between Friends?

Written by: Brian Botham, CDS

Enough to part you from half a week’s pay, if you’re not careful. Before you can determine if your pushrod is within the required limit you must verify what type of brake chamber you have. Typically most trucks and trailers will have a Type 30 chamber on the drive and trailer axles. Steer axles are usually Type 20 or 24. Each size and type of brake chamber has a maximum allowable stroke limit. The chart below lists a few possible combinations, but there are many more. If in doubt, ask your fleet maintenance supervisor what the stroke limit for your chamber size and type is, and never let the pushrods get beyond that.

Automatic slack adjusters aren’t supposed to go out of adjustment. When they do, there’s something wrong somewhere.

What do you mean my brakes are out of adjustment? I have automatic slack adjusters; that’s not supposed to happen. But it does and with alarming frequency. The number one out-of-service defect found during roadside inspections continues to be brakes out-of-adjustment – and auto-slacks aren’t immune from the problem. They are often found to be out of adjustment, but the fix isn’t a 9/16 wrench.

Back in the good old days, brake adjustments were routine for most drivers. We’d crawl under the truck or trailer, put the wrench on the slack adjuster and tighten it up; then we’d back it off half a turn and give the slack a little tap so the lock ring popped out and we were set. Today though, we are all running auto-slacks, so the need to adjust them is no longer there, right? Well, sort of. Let’s take a closer look.

Automatic slack adjusters are supposed to adjust themselves automatically, just as the name implies. And all things being equal, that’s what happens – most of the time. When an auto-slack comes up out-of-adjustment, there’s a reason – usually one of two reasons actually.

One, the slack adjuster is defective; or two, the adjustment mechanism is not being given the opportunity to perform.

A defective slack may not appear broken, but if it’s slipping and not maintaining the proper pushrod stroke – it’s broken. When you write up a defective auto-slack, don’t be satisfied with having a mechanic “readjust” it for you. It’ll likely go out of adjustment again by the third or forth brake application. In fact, a manual readjustment of a defective auto-slack is about the last thing you want to do. The National Transportation Safety Board (NTSB) condemns Auto-slack Adjustment Process.

Let’s clear up a few issues on auto slacks and how they work. The adjustment mechanism works by sensing the length of the stroke – either the application stroke or the return stroke. There’s a ratchet device inside the slack that clicks over if the stroke is longer than it should be, but it takes considerable force to click the ratchet over. The more force you apply to the brake pedal, the more opportunity the brakes have to adjust themselves.

Ironically, it’s often our better drivers who get caught with over-stroking brakes. Drivers today are taught defensive driving skills, which include leaving enough space for stopping, reducing following distances, etc. So more and more, we see drivers who require only light brake applications while stopping. The problem with this scenario is that the slack adjusters are not receiving enough air pressure to force an auto-adjustment. In other words a good defensive driver who anticipates problems before they happen and doesn’t make hard brake applications may actually be a detriment to self-adjusting slacks.

The Solution is Under Your Foot.

So what can you do to combat this problem? The answer is very simple! A twelve-pack! And I don’t mean your favorite wobbly-pop. What I mean is 12 solid brake applications at the beginning of your day.

With your brakes released and the system charged with air, make sure your wheels are chocked, or the truck is in gear, if you are running a multi axle trailer make sure all axles are in the down position, otherwise they will not adjust. You should start with full air tanks -- about 125 psi. Make 12 full-pressure brake applications. You will probably run out of air pressure after the first four or five, so be prepared to start up and refill the tanks. This procedure will ensure you have applied enough force to the adjuster to click the ratchet over. You should do this procedure every day to ensure that your slack adjusters are doing their job.

But what if they still go out of adjustment? Better question, first things first: how will you know if they’re maintaining their adjustment?

The only way to check proper adjustment and pushrod travel is by visually checking them regularly.

“Mark and Measure” is how the DOT and a mechanic will check your pushrod travel and it’s how you should also check pushrod travel. With your wheels chocked (front and back) and brakes released, all axles down and your air pressure between 90-100psi mark your pushrods in their resting position, against the brake chamber. Mark the pushrod with a piece of tire chalk or soap stone, or you can put a zip tie on the pushrod. Now make and hold a full brake application and recheck the mark on the pushrod. If it’s within the prescribed limits, you’re away to the races.

You may need someone to do this for you or you can use a “brake buddy”, which is a bar or piece of wood that goes in between the steering wheel and your brake pedal. If the truck is equipped with brake stroke indicators, you might not have to get on you back under the truck to check the brake, but occasionally, the stroke indicators need to be checked to ensure they haven’t slipped out of position.

Should you discover an auto-slack that’s not maintaining the proper stroke, don’t seek to have it readjusted, write it up as defective. Follow through if you can to ensure the next driver on the truck won’t be stuck with a defective brake.

Really, I can’t emphasize this enough: Don’t walk away satisfied that the problem has been taken care of just because you wrote it up. The story I mentioned earlier made reference to mechanics and technicians automatically assuming that an auto-slack that is not readjusting properly just needs to be readjusted. This is an industry-wide misconception that must be addressed. Unfortunately, it’s not the mechanic that gets the ticket for improperly adjusted brakes, or worse, charged with failing to maintain or failing to do a pre-trip: it’s you, and the solution has to start with you.

Remember; never put a wrench on an automatic slack adjuster. If it is out of adjustment there is an underlying cause. Trying to adjust an auto-slack can have grave consequences. A driver adjusted his auto slacks and the resulting crash cost two innocent people their lives.

Brake Adjustment Limits – Clamp Type Brake Chambers

Maximum pushrod stroke with 90-100 psi system pressure, and full brake application.

Size & Type Marking Outside Diameter Adjustment Limit
20 None 6 25/32" (172mm) 1 3/4" (45mm)
20L L Stamped in cover, Stroke Tag 6 25/32" (172mm) 2" (51mm)
24 None 7 7/32" (183mm) 1 3/4" (45mm)
24L L Stamped in cover, Stroke Tag 7 7/32" (183mm) 2" (51mm)
24LS Square ports, Tag & Cover Mark 7 7/32" (183mm) 2 1/2" (64mm)
30 None 8 3/32" (205mm) 2" (51mm)
30LS Square ports, Tag & Cover Mark 8 3/32" (205mm) 2 1/2" (64mm)
36 None 9" (228mm) 2 1/4" (57mm)