Question:
Something some of the students in my class could not agree on was when to do the air brake check.
I never got to ask my instructor so maybe someone can help answer.
Should I do the air brake check before or after I hook up to the trailer? To me it makes sense, if you have to test the trailer brakes, you should test them at the same time as the tractor brakes.
But what if you're bobtailing a greater distance to your trailer, say a couple miles. Should I test the tractor brakes in this scenario?
Thanks!

Answer:
qnce you have hooked to a trailer and done your tug test hook up your air lines and light cord then release your brake to the trailer let the air build up to normal 120 psi then do your brake test. if you think about it how many times are you going to pretrip your truck with no trailer .
YOU CAN SLEEP AFTER YOU DELIVER THE LOAD

Answer:
As a insturctor I teach my students' several air brake test. You have have three brake systems on a tractor and trailer. Parking Brakes, Service Brakes and Emergency Spring Brakes. The first brake system to check, is the parking brakes.
When you start your truck you let the air build up to between 100-125psi, until the gov. cuts out. This will vary on the manufacturer seting. With the tractor protection valve set or pulled out, put the truck in 1st gear, ease out on the clutch half-way and remove your foot off the foot brake, if the parking brakes are adjusted correctly then the truck should not move, if the truck moves get the brakes adjusted.
Once the gov. has cut out above 100psi. you should do a service brake pull test on the tractor only. This is done by putting the truck in 1st. and releasing the tractor protection valve.. Release the clutch half-way, but leave your foot pressed down on the foot brake, this is the service brake system. If the brakes are adjusted properly the tractor will not move. If the truck moves get the service brakes adjusted.
The next brake check should be the emergency spring brake check. Release the tractor protection valve. Fan off the air and at around 60psi. the low air pressure warning light will activate. Continue to fan off the air until around 40psi. and the tractor protection valve will pop. Now start the truck up and while the air pressure is below 100psi. put the truck in 1st and ease out on the clutch half-way with your foot off the service foot brake. If the emergency spring brakes are adjusted properly the truck will not move. If the truck moves get the emergency spring brakes adjusted.
The next brake check is the slack adjuster brake pull check. Once the air pressure is past 100psi, release the tractor protection vlave and put the truck in 1st. Release your foot off the service brake and put the tip of your fingers under the steering wheel at the 9:00 and 3:00 o'clock positions applying only light pressure. Accelerate to 5mph, once at 5mph push in the clutch and brake hard. If the tuck pulls to one side or the other then you have slack adjuster out of adjustment. If the truck pulls to the right then the slack adjusters' are out of adjustment on the left. If the truck pulls to the left then the slack adjusters' are out of adjustment on the right side of the truck, get them adjusted.
You can repeat the same pull test with the trailer on the three brake systems as outlined above. Remember to also release the trailer spring brake push pull valve, along with the tractor protection valve when doing the service brake pull test. If your truck has a hand brake for the trialer apply it and remove your foot from the service foot brake to check the trailer service brakes, the hand valve is now the service brake for the trailer. It is important to leave the spring brake trailer valve set on the trialer and release the tractor protection valve only when doing the emergency spring brake check. You now can do the slack adjuster pull test the same way as you did for your truck, just rember to release both brake valves when doing so.
Now let me explain why it is important to do the brake pull test with the tractor unhooked. You could actually drive across country with a load and be unaware of the fact that your truck has a defective brake system. The trailer brakes will mask your out of adjustment, on your truck braking systems. By doing the brake pull test before you hookup, gives you an accurate idea of the brake adjustment on the truck, it is very important that your brakes are working together and applying pressure evenly on both the tractor and trailer. If you do the brake checks with the trialer hooked, then the trailer brakes could be doing all of the work. Now your going down a mountian grade and your brakes are over heating. You have just discovered that your truck brakes are not adjusted properly and that all you have are trailer brakes. You will never make it down the grade because the trailer brakes will fade and you will be out of control.
I hope this helps all of you new drivers, catch you on the flip.

Answer:
Thank you for the responses. I did not know about the brake check of driving forward and applying the brakes to see if one side is out of adjustment.
I do know the tug test, i do this with the tractor brake closed and then with the trailer brake closed.
Then I check the spring brakes by fanning the brake.
I was concerned on when I should do the spring brake check. It makes sense to me to be connected to the trailer, so that while you are fanning off the brakes, first a warning signal should appear or sound, then at 25-40 psi the trailer brakes should close and between 20-25 psi, the tractor brakes should close (usually they both pop out at the same time) But if I was not connected to the trailer yet, I would have to perform this test for the tractor and then back up to the trailer and perform the test again to test the trailer spring brakes.
SO, I need to make sure that being connected to the trailer when fanning off the brakes is accurate. Some of the students argued with me about this to no end and I would just like to know who is right.
Thanks.

Answer:
I have always performed the foot brake test and tractor parking brake test whenever I get in a tractor - every time. If I am taking that tractor bobtail out on a public road, I will also do the pumpdown test to make sure the alarm and button work corrctly. If I am just driving that tractor to another area of the yard to get a trailer, I skip the pumpdown until after I am connected to the trailer, and then I perform the entire set of tests. My theory is that I know the foot brake works, and that is all I am apt to need going in third or fourth gear across the yard, but I need to be sure of everything before I pull on to a public road.
PP
Keep electing the same kinds of politicians and you will keep getting the same kind of government.
Answer:
Question: Should I do the air brake fan test with the tractor and trailer hooked.
You should do all of the pull test as I outlined in my previous post. Do the tractor and trailer sepearate, this is criticial to ensure that your brake adjustments are set properly for the tractor, because the trailer may mask the braking effect.
Then do the fan test again with the trailer and tractor hooked. Remember always error on the side of safety, you have a 80,000lbs vehicle that needs to have its full braking effect at all times. It is just as important to be able to stop going down a city street, as a mountian grade. What if your in a city and a child runs out in front of your truck, that stoping distance may mean the difference between life and death.
Catch you on the flip.

Answer:
Thank you scalehouse for your reply.
It is very helpful. And just the answer I needed.
Thank you!

Answer:
Hope you had a good holiday. One of these days I will try to stop by and shoot the breeze with ya when I am home during the week.
DRIVER BY CHOICE
Buzzdog