Emergency brakes are activated my depressing the pedal or pulling the lever which then causes the cable to pull on the brake mechanism. As the brakes wear and the cable stretches, it gets out of adjustement.
When I used to work on these rigs, we'd crawl under the rig and adjust the cable tensioner in order to achieve good emergency brake operation. There are a couple of tests you can perform after it's adjusted. 1. Start the engine while parked on a flat surface. Put the rig in drive while at idle and take your foot off the main brake pedal. If the emergency brake is fully engaged, it should hold the rig from rolling forward. 2. Drive the rig at 10 miles per hour and apply the emergency brake. It should stop the rig in a reasonable distance.
In 1976, I was driving on an expressway when the traffic came to a stop. I applied my main brakes and the master cylinder failed which caused a total loss of braking. I immediately deployed the emergency brake and was able to stop just before I hit the car in front of me. A well adjusted emergency brake can be a life saver.
Jim