Using the hand valve while driving can lock the trailer wheels and make the trailer skid, which is how a jackknife begins. The brake pedal sends air to the brakes on every unit at once, so use it for normal stops and save the hand valve for testing the trailer brakes.
For this test, charge the trailer brakes, shut off the engine, and pump the brake pedal to bring the air pressure down. If the valve works, the trailer air supply knob, also known as the tractor protection valve control, should pop out (or switch to "emergency"), usually when pressure drops to somewhere between 20 and 45 psi.
The service line carries air only when the brakes are applied, so a break in it goes unnoticed at first. Once you brake, air escapes through the open line, tank pressure drops, and the trailer emergency brakes may come on.
Before backing, the tractor should sit squarely in front of the trailer. Approaching at an angle can shove the trailer sideways and break its landing gear.
The jaws have to wrap around the shank, the narrow neck of the kingpin, and not around its wider head. Jaws closed on the head are not truly locked, and the trailer could come loose.
A good coupling shows the two fifth-wheel plates flush, with no space between them, and the jaws locked around the kingpin. A gap can mean the kingpin is riding on top of closed jaws, and the trailer could break loose.
If the lever stays unlocked, the jaws may not be holding the kingpin securely, and the trailer could break away once the rig is moving. Do not drive until the coupling has been corrected.
A gentle forward tug against locked trailer brakes shows whether the fifth wheel jaws are truly holding the kingpin. If the coupling has not locked, the tractor will start to separate from the trailer.
Park with the spring brakes set, and if the trailer has no spring brakes, chock its wheels. Never park using the trailer hand valve: it is meant only for testing the trailer brakes, and if the air leaks away, the brakes it applied can release.
Shut the engine off so you can hear, then set and release the trailer brakes; you should hear the brakes move as they apply and air rush out as they release. Silence suggests the lines may be crossed.
On newer vehicles the trailer air supply control is a red, eight-sided knob. Pushing it in sends air to the trailer, and pulling it out cuts that air off and applies the trailer's emergency brakes.
Trailers built before 1975 were not required to have spring brakes, so many older units depend only on air stored in the trailer's own air tank for emergency braking. If that air leaks away, nothing automatically holds the trailer in place, and it could roll.
Crossed lines send supply air into the service line, so the trailer tanks never fill. A trailer with spring brakes would stay braked and alert you, but an older trailer without them can roll freely, letting you pull away with no trailer brakes.
Glad hands are the fittings on the service and emergency air lines that lock onto matching fittings on the trailer, carrying air from the tractor to the trailer brakes. They are not used to warn traffic or to secure cargo.
At the correct height, the fifth wheel raises the trailer a little as the tractor slides beneath it. If the trailer sits too low, the tractor can strike and damage its front end; if it sits too high, it may not couple correctly.
With the trailer a touch lower than the middle of the fifth wheel, the dolly picks it up slightly as it goes under. A trailer that sits too high may not couple correctly, while one that is far too low can be struck and damaged.
Coupling the glad hands to each other or to dummy couplers seals their openings, so road grime, moisture and debris cannot get into the air lines and cause brake trouble.
Hooking up the air lines first lets you send air to the trailer and check its brakes before the tractor goes under. The electrical cord is plugged in later, after the coupling has been locked and inspected.
Locked trailer brakes keep the trailer from rolling away while the tractor slides under it, so the fifth wheel can catch the kingpin. You set them by pulling out the air supply knob or moving the tractor protection valve control from normal to emergency.
Rolling forward slowly before you use the hand valve (trolley valve) on the trailer brakes lets you feel whether they take hold, which shows the trailer brakes are connected and working. That only works once air pressure is normal and the parking brakes are off.
Manufacturers typically set the valve to close somewhere between 20 and 45 psi. When it closes, it holds the remaining air in the tractor (or truck) brake system, so a trailer that breaks away or leaks badly cannot drain it.
The hand valve holds the trailer with air pressure, and if that air leaks away the brakes let go and the rig can roll. Always use the parking brakes when you park.
Landing gear left partly down can catch on railroad tracks or other objects in the road, so crank it fully up and secure the handle before you drive.
The trailer brakes need a full supply of air to work properly during coupling, so let the pressure build to its normal level first. With low pressure, the brakes may not apply or hold the way they should.
When the trailer air supply control is pushed in, air flows through the emergency (supply) line and fills the trailer tanks. Pulling the control out does the reverse: it cuts off the trailer's air and sets the trailer emergency brakes.
Glad hands are the couplers that connect the service and emergency air lines between the tractor or truck and the trailer. A rubber seal in each one keeps air from leaking at the connection.
Without enough grease, friction builds up between the tractor and the trailer, and that can make the rig hard to steer, especially in turns.
By common convention the red line is the emergency (supply) line and the blue one is the service (control) line. Mixing them up when coupling can keep the trailer brakes from working properly, so check each connection.
Every question in this set has its own page with the correct answer and an explanation.
