Here's an explanation on Spring Brakes, the Mother of all parking brakes. When doing a pre-trip the springs must on and tested for holding power, you give the throttle [engine running in gear] a quick hit with your foot.
Don't put your pedal to the metal 'cause nothing is going to hold back that kind of torque! The safety factor is much higher with modern spring brakes because the highly compressed spring is now factory clamped so it's not serviceable meaning changing a blown diaphragm so any danger is taken away. [years ago they were serviceable and there were fatalities from the spring letting go from the caging bolt or untrained people removing the clamp without caging the spring]
Check out this pdf below, it explains in great detail the proper servicing procedures.  Thanks to Mac over at http://skoolie.net
Spring Brakes PDF Instructions
Here's a previous post I made about Spring Brake assembly replacement: How To Replace A Spring Brake Assembly
There are ways to tell if a spring is broken inside the chamber. With the springs released give the chamber a wack with the palm of your hand and you will hear a rattle.
When performing a tug test you may notice that movement does not take much engine torque with the springs applied. You just need a very slight hit on the throttle pedal for this test. I've heard of a few operators getting up to 1500 rpm with the transmission in gear when doing a tug test.
They automatically assume the springs aren't holding. Nothing is going to hold back that kind of torque and it's a quick way to twist the driveshaft out of shape.
Spring Brakes - Precautions To Take When Servicing
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