Brake adjustments

You measure the applied stroke of the pushrod with a minimum of 90 PSI application pressure.

Drive axles are most often type 30 chambers. Anything in excess of 2" of pushrod stroke is out of adjustment. Typically a properly setup slack adjuster/pushrod will keep the adjustment to somewhere between 1-1/2" to 1-3/4".

The steer axles use a smaller chamber and those usually have shorter stroke.

 

She didn’t do you any favors. Only one brake out of adjustment isn’t criteria for an OOS order. They got me once for that. I had both fronts set at 2”. I had just done the brakes myself. Had no idea a 20 chamber max travel is only 1.75”. Put me OOS, till I adjusted them right. Oops!!
You can mark the pushrod at the can, using a chalk stick, or a piece of tape, with brakes released, and pry it at the pushrod/slack adjuster, and measure the mark when it’s out brake applied. Much easier to have a helper hit the brakes, while you measure the travel. The old manuals used to back them off at 3/8”-1/2” on drives, and 5/8”-3/4” on steers. Common practice was to back the fronts off more, to avoid them ever locking up, or grabbing first in winter. The new ABS systems and automatic slacks suggest backing them off about 3/8” -1/2”. A lot of Carriers will back them all off 1/4”. That’s what I do. Though I still back the fronts off a bit more, just in case, about a 1/2”. Afterwards I make sure it rolls freely, and none are dragging. The automatics will adjust themselves anyway if working properly. Main thing is they’re equal. Shouldn’t need adjustment afterwards. If one does, the slack adjusters bad. Adjusting them is simple. Just tighten a small usually square bolt on the slack, till it’s tight, it only goes one way, you’ll see little spring loaded button above it popping in and out, as you’re tightening it. Once it stops turning, it’s tight shoes are against the drum. Simply pry up the button/lock to release it and back off 3/8”. Release the spring loaded button making sure it locks back in Some slacks just tighten up, and back off. No locking button to pry up. They tighten up easier than they back off, making a much louder clicking noise when backing off. Sounds like you’re breaking something, Lol. Either way, it’s easy to figure out. Just watch the shoes, they’ll tighten up against the drum, you’ll know your going the right way. YouTube has some good examples.

 

The best way is to make sure you grease the auto slack adjusters and before you shut down for the night, before you set the parking brake, hold the brakes to the floor for one second 5 times.

Occasionally (every 6 months as an example) you can turn the slack adjuster bolt (on the slack adjuster not the pushrod) until it is tight then back off half a turn.

That is hard on a slack adjuster so if you have to do it often you either need to see if greasing can free them up or get your slack adjuster replaced.

With modern brakes being out of adjustment is an indicator the slack adjusters are working. Typically when you stop the drum will be cool or you will see rust on the drum surface.

If you drive in the dirt or drive through states that use lots of brine for snow control you may actually need to grease the slack adjusters every week.

Because I am in the mountains I do this myself even as a driver. I have had too many pucker factor incidents where a truck will pull to one side when I get cut off to be complacent about them anymore.

When I get in someone else's truck I do the brake to the floor for a second a few times after it builds air. Often you can hear them click away as most drivers do not.

 

Included an image to show where the adjusting nut is.

Note that your spring brake needs to be released and you need more than 80psi in the air tanks.

Tighten until you feel resistance and back off a turn is almost universal among slack adjuster adjustments.

But the adjustments should be rare, if you have to do this often it is time to take the truck to the shop

 

Measure the space between the brake shoes and the drum. If you have automatic slack adjusters, you might as well let them try to adjust them self by stopping, setting your trailer brakes. then press the brake pedal as far as possible and then release. Repeat this 5-7 times. If the adjusters are working your brakes shoes will wind up about 1/4 inch from the brake drum when the brakes are released. You can always turn the adjusting bolt down until the shoe is tight against the drum and then loosen that adjusting bolt half a turn. I think the adjusting bolt is 7/16 I always used a cresent wrench. The bolt has a spring-loaded guard that you have to overcome. Think of a box-end wrench of the proper size. You put the wrench on the bolt head. Push the wrench even further down on the bolt head and you will push the spring guard out of the way and you can turn the bolt. Turn right to tighten and left to loosen.

 

1/4" is a ton of clearance. I can't remember the exact numbers but something crazy small like the thickness of a few sheets of paper worth of shoe to drum clearance will equate to something like an additional 1/4" of pushrod travel.

That's why drum brakes are so suceptible to brake fade. Drums heat up and expand and the chambers simply run out of stroke. If you're already at the max 2" of stroke, you don't have much margin left.

 

While stroke is the way DOT measures adjustment, a quick eyeball check is to look at the angle of the pushrod & slack adjuster. They should make a ~90° angle at max application. That maximizes the force transfer from the pushrod to the cam. Over a 90° angle means they're adjusted too tight, under 90° means too loose. Although slight over 90° isn't bad if they're not dragging, as it keeps the brake near max effectiveness longer as the drum expands as it heats up.

 

Look at the gap between shoe and drum.

There shouldn't be any.

Any gaps will make excessively longer strokes.

Can't explain it any easier then that.

 

Buy these, follow the instructions

EZ Chek - Air Brake Stroke Indicators


Checking brake clearance should be a skills test for every cdl license holder.