Brake Adjustment/Stroke

S-Cam Type Air Brakes

The first thing you need to know is what type of brake chamber and what type of slack adjuster you are working with. Automatic slack adjusters are required on vehicles manufactured on and after May 31, 1996. 

The following picture is of a manual slack adjuster. Your probably not going to see to many of these but there still out there.

This type of slack adjuster can be adjusted with a 9/16″ wrench by pushing in the spring loaded locking sleeve and turning the adjusting nut until the brake shoes contact the brake drum. From there back the adjusting nut off 1/3 to 1/2 turn, make sure the spring loaded locking sleeve pops back out. 

Make sure your turning the adjusting nut in the right direction. The s-cam must rotate in the same direction as a brake application.

In Ontario, drivers can become certified to perform brake re-adjustments on manual slack adjusters. Only those who have obtained certification are permitted to perform brake re-adjustments. This certification does not permit you to manually re-adjust automatic slack adjusters.

There are various types of automatic slack adjusters and you need to know what your working with in order to adjust them properly. Rockwell slacks will have a pawl that you need to release in order to back them off others may ratchet. Below are a couple of different types the one on the left is the Rockwell that requires the pawl to be released by prying up on it with a screwdriver in order to back it off. 

How to tell the difference between long stroke brake chambers and regular stroke brake chambers. Many manufacturers of long stroke chambers identify them by installing a tag in that shows the rated stroke for the chamber. These tags are usually installed near the air fitting or a clamp bolt. Information on the tag may also be repeated on the chamber itself. Most long stroke brake chamber manufacturers identify the chamber by using square shaped ports where the air fittings connect. In many cases, the square shape has rounded corners. 

Here are the steps you must take to measure applied pushrod stroke.

  1. Secure the vehicle with wheel chocks or blocks.
  2. Ensure air pressure is above 621 kpa (90 psi) and release the spring brakes.
  3. Select one of the following methods:
    • Method 1: Mark the pushrod at the brake chamber or at a suitable fixed reference point. (Use chalk, soapstone, marker or other similar instrument. Marks must be narrow and precise.)
    • Method 2: Measure the released position of the pushrod. (Measure and note the distance from a point on the pushrod to a suitable fixed point at the brake chamber. This is measurement number 1.)
  4. Raise or lower the air pressure by running the engine or pumping the brake pedal until both the primary and secondary air-tank gauges display 621 to 690 kpa (90 to 100 psi).
  5. Shut off the engine.
  6. Press and hold the brake pedal in the fully applied position, and use a suitable means to hold the brakes applied in order to leave the cab and inspect them.
  7. Determine the applied pushrod stroke. (Continue to use the previously selected method.)
    • Method 1: Measure the distance from the brake chamber or fixed reference point to the mark on the pushrod.
    • Method 2: Measure the applied position of the pushrod. (Re-measure and note the distance from the previously selected point on the pushrod to the previously selected fixed point at the brake chamber. This is measurement number 2.) Subtract measurement 1 from measurement 2 to calculate the applied pushrod stroke measurement.
  8. Determine the number size (such as 16, 20, 24 or 30) and type (such as standard or long-stroke) of the brake chamber.
  9. Determine the adjustment limit for the brake chamber.
  10. Compare the applied pushrod stroke to the applicable adjustment limit and identify any brake that exceeds the adjustment limit as defective.

Below is an interesting tool for identifying brake chamber type, takes the guess work out of it. Not promoting there website but they offer a few interesting tools.  

Type Outside DiameterRated StrokeMaximum Legal Stroke Limit
* Note: If type 36 chamber is used, slack length should be less than 6".
Type Outside DiameterRated StrokeMaximum Legal Stroke Limit
* Note: Identified by square air port bosses.

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