The tractor protection valve keeps air in the tractor  or truck brake system should the trailer break away  or develop a bad leak. The tractor protection valve  is controlled by the "trailer air supply" control valve  Never use the hand valve for parking because all  the air might leak out unlocking the brakes (in  trailers that don't have spring brakes). Always use  the parking brakes when parking. If the trailer does  not have spring brakes, use wheel chocks to keep  the trailer from moving.  The trailer hand valve (also called the trolley valve  or Johnson bar) works the trailer brakes. The trailer  hand valve should be used only to test the trailer  brakes. Do not use it in driving because of the  danger of making the trailer skid. The foot brake  sends air to all of the brakes on the vehicle  (including the trailer(s)). There is much less danger  of causing a skid or jackknife when using just the  foot brake.  Look at Your Path. Look at your line of travel  before you begin. Get out and walk around the  vehicle. Check your clearance to the sides and  overhead, in and near the path your vehicle.  Use Mirrors on Both Sides. Check the outside  mirrors on both sides frequently. Get out of the  vehicle and re-inspect your path if you are unsure.  Back Slowly. This will let you make corrections  before you get too far off course.  Correct Drift Immediately. As soon as you see  the trailer getting off the proper path, correct it by  turning the top of the steering wheel in the  direction of the drift.  Pull Forward. When backing a trailer, make pull- ups to re-position your vehicle as needed.  6.2.2 – Tractor Protection Valve  6.2 – Combination Vehicle Air Brakes  6.2.1 – Trailer Hand Valve  Subsection 6.1  Test Your Knowledge  These questions may be on your test. If you can’t  answer them all, re-read subsection 6.1.  1.  What two things are important to prevent  rollover?  2.  When you turn suddenly while pulling  doubles, which trailer is most likely to turn  over?  3.  Why should you not use the trailer hand brake  to straighten out a jackknifing trailer?  4.  What is offtracking?  5.  When you back a trailer, you should position  your vehicle so you can back in a curved path  to the driver’s side. True or False?  6.  What type of trailers can get stuck on railroad- highway crossings?  2005 Model Commercial Driver’s License Manual  Figure 6.5  You should study Section 5: Air Brakes before  reading this. In combination vehicles the braking  system has parts to control the trailer brakes, in  addition to the parts described in Section 5. These  parts are described below.  Section 6 – Combination Vehicles Page 6-4 