dbstr is correct in a way as the rules only apply to the TP valve. The park brake might be considered a "similar device". But on the other hand all equipment has to operate as designed or it is considered malfunctioning equipment.
If the student touched the button after the 20 psi threshold, it shouldn't matter because the button already failed. I would of pointed that out to the instructor. Hey that button should of popped out by 20 psi and don't touch it.
Another reason you get three tries is so you can learn from your mistakes. A good instructor would explain why you failed and the go over the correct procedure.
I failed my test
Discussion in 'Questions From New Drivers' started by dano1971, Dec 1, 2010.
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As I mentioned the park brake button is not required to pop and in fact the CVSA issued a safety bulletin in 1997 concerning this. Various states were placing trucks out of service because of the failure of the yellow button to pop. These states were in error as there is no such requirement for the park brake button to pop, and the only place they could find where it was stated to be required was the CDL manual.
http://www.bendixvrc.com/itemDisplay.asp?documentID=5194
In the case of the TPV, it's function is to protect the tractor air supply in the case of a trailer air loss or loss of the trailer. As your quote stated in part "the service brakes on the towing vehicle will be capable of stopping the towing vehicle", after a breakaway or trailer air loss. If the park brake button pops, service brakes are no longer an option.
A major problem with the TPV test as in the CDL manual is that a functional TPV can either pass or fail the test as stated in the manual, and a defective TTV can also pass or fail the test. Makes for a rather pointless test, other than to have CDL applicants jump through a meaningless hoop.
Although not required by regulation to pop out, the park brake button is designed to do so at a pressure stipulated by the vehicle manufacturer, (and made by the valve manufacturer). Most vehicle manufacturers specify the trip pressures allowing for at least a 5 psi difference between when the TPV closes and the trailer air supply pops out and when the Park brake button pops out.
For instance they will state that the trailer spring brakes will apply when air pressure drops to between 35-45 psi, and the tractor spring brakes will apply between 20-30 psi. Note the minimum 5 psi difference between them.
The most positive TPV test is the CVSA test. http://www.cvsa.org/programs/documents/nas/2010-01 Tractor Protection Systems Revised 05-19-10.pdf
The TPV test in chapter 6 states (Reasonably correct):
Test Tractor Protection Valve. Charge the trailer air brake system. (That is, build up normal air pressure and push the "air supply" knob in.) Shut the engine off. Step on and off the brake pedal several times to reduce the air pressure in the tanks. The trailer air supply control (also called the tractor protection valve control) should pop out (or go from "normal" to "emergency" position) when the air pressure falls into the pressure range specified by the manufacturer. (Usually within the range of 20 to 45 psi.)
If the tractor protection valve doesn't work right, an air hose or trailer brake leak could drain all the air from the tractor. This would cause the emergency brakes to come on, with possible loss of control.
While Chapter 5 states:
Check That Spring Brakes Come On Automatically.
Continue to fan off the air pressure by stepping on and off the brake pedal to reduce tank pressure. The tractor protection valve and parking brake valve should close (pop out) on a tractor-trailer combination vehicle
It appears they are asking for the valves to simultaneously. If so this is wrong (And dangerous if allowed) and a direct contradiction to the chapter 6 quote above. Additionally (Being petty here) The TP valve does not pop out, the TP control valve pops out. The TP valve is usually under and toward the rear of the cab or sleeper. A TPV closes and can be closed, but can't pop out!
Best regardsdano1971 Thanks this. -
I'm getting to Re-test on the 14th of December at 9:00am, my school instructor got me a sooner appointment. Thank GOD
123456, Rerun8963 and Buckeye 'bedder Thank this.
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