Pumping service brake with parking brake ON?

We were taught to put the truck in 1st gear, shut the truck off, turn the key to on without starting the engine then do you air brake test. Something about too much air pressure blowing something up.

 

ou are doing what is called, "compounding the brakes"

If you do that in front of a DMV tester on your day for the big test, he may send you home, as you're done

You are actually supposed to chock the wheels, release the brakes, pump down, till the low air warning light comes on, then continue pumping till the brake knob pops out.

Today's computerized trucks may actually prevent compounding of the brakes, but I can assure you that a DMV tester will fail you.

When you compound the brakes, you are adding MORE air into the system. You run the risk of blowing out safety valve, lines, and even a brake can or two.

So DO NOT do that.

 

Yep, there are several key actions and functions to perform to do a low air warning test (as well as emergency spring brake systems) ... and the thing is, each action is very vital. So it might have been good if the instructor bothered to make sure the entire class understood these functions and the fact you can't do it one way or another way ... it must be done only one way.

(This is part of truck driver training where you don't want to be absent that day and need to stay awake and take proper notes)

 

When the brakes are 'set' the springs are sprung all the way out. So what happens when you pump on the service brakes you're basically putting more pressure on the springs to expand when they already are out. That and the slack adjusters are already pushed out with the S cam seating the brake pads against the drums. So it's an all around bad deal. I've owned my truck for a little over 4 years after running just 8 months as a company driver, a few times in my rookie years I recall pumping the brake tests with the brakes set. Long story short, last year I had to replace all the drive slack adjusters, I believe I had done the damage by pumping the service brakes while brakes were set.

 

Our instructor made it very clear. Pump the service brakes with air brakes applied and we would finish out the day at the house.

 

Trucks have an anti-compounding valve. Sometimes they fail. When they do, they like to break s-cam housings, then that brake is effectively ####ed.

 

Whose trucks have those? Neither of mine do. Pump with the park on n I'll be buying brake parts

 

What year are your trucks? They've been pretty standard fare for a while, although I'm not sure exactly when they came about.
Trying to find that out didn't yield a definite answer, but I found a video I'll share explaining how it works, for anyone interested

 

1995 1997. I bet new ones have all kinds of fancy upgrades.