Testing Low Pressure Warning Signal

Newbie here...

Would like to have a better understanding about the air brake low pressure system and test. On systems containing both a Primary and Secondary system, I assume each system has it's own separate path to the low pressure warning signal. So when testing, unless both systems leak down at the exact same rate, we're really only testing the side that leaks down the fastest to the signal point (60 psi). The system leaking down the slowest during the test never actually gets tested against the low pressure warning signal device. It seems to me that we are merely testing one of the two systems, and of course that the audible part of the warning system is functional. How will you ever know if, after a failure, that the other system will actually trigger the audible warning signal?

Or do I have it all wrong? Are there two low pressure signal devices (one for Primary, one for Secondary) each with warning tones at different frequencies?

Thanks.

You have two gauges, two signals and each operate independently. Primary gauge is for the air supply to the tractor, secondary air supply to the trailer. You also have release valves for each one. So, yes, you can release or engage either set of brakes by themselves, and you can also test each set of brakes separately from the other.

Dual air brake systems:

Most newer heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has two separate air brake systems which use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc. One system typically operates the regular brakes on the rear axle or axles. The other system operates the regular brakes on the front axle and possibly one rear axle. Both systems supply air to the trailer, if there is one. The first system is called the primary system and the other is called the secondary system. Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of 100 psi (690 kPa) pressure in both the primary and secondary systems. Watch the primary and secondary air pressure gauges (or needles, if the system has two needles in one gauge). The low air pressure warning light and buzzer should shut off when air pressure in both systems rises to a value set by the manufacturer. This value must be greater than 60 psi or 410 kPa. The warning system devices should come on before the air pressure drops below 60 psi in either system. If one air system is very low on pressure, either the front or the rear brakes will not be operating fully. This means it will take you longer to stop. Bring the vehicle to a safe stop and have the air brake system fixed.

This link might help with your understanding:

Air brake systems

Much of this information can be found on Trucking Truth's High Road Training Program. This training is far more comprehensive and effective than repetitive reading and memorizing of the State CDL Manual.

I think it depends on the truck actually. I'll have to pay attention to this tomorrow when I do my pretrip to see how it is on these trucks, but I'm pretty sure I've been in trucks where there are two slightly different pitched alarms. I think most trucks just have one alarm that goes off when one of the air pressure systems gets too low.