Air brake alarm at stop sign

Here's the procedure (copy and paste) for the DOT air brake system checks. I know all of this won't apply to your coach since you do not have a trailer being towed but maybe it will help you find if you have a system leak or maybe it's just the governor.

Test Low Pressure Warning Signal. Shut the
engine off when you have enough air pressure so
that the low pressure warning signal is not on. Turn
the electrical power on and step on and off the
brake pedal to reduce air tank pressure. The low
air pressure warning signal must come on before
the pressure drops to less than 60 psi in the air
tank (or tank with the lowest air pressure, in dual
air systems).

If the warning signal doesn't work, you could lose
air pressure and you would not know it. This could
cause sudden emergency braking in a singlecircuit
air system. In dual systems the stopping
distance will be increased. Only limited braking can
be done before the spring brakes come on.
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 and the
parking brake valve should close (pop out) on
other combination and single vehicle types when
the air pressure falls to the manufacturer’s
specification (20 – 45 psi). This will cause the
spring brakes to come on.

Check Rate of Air Pressure Buildup. When the
engine is at operating rpms, the pressure should
build from 85 to 100 psi within 45 seconds in dual
air systems. (If the vehicle has larger than
minimum air tanks, the buildup time can be longer
and still be safe. Check the manufacturer's
specifications.) In single air systems (pre-1975),
typical requirements are pressure build-up from 50
to 90 psi within 3 minutes with the engine at an idle
speed of 600-900 rpms.
If air pressure does not build up fast enough, your
pressure may drop too low during driving, requiring
an emergency stop. Don't drive until you get the
problem fixed.

Test Air Leakage Rate. With a fully-charged air
system (typically 125 psi), turn off the engine,
release the parking brake (push in); and time the
air pressure drop. The loss rate should be less
than two psi in one minute for single vehicles and
less than three psi in one minute for combination
vehicles.

With the air pressure built up to governor cutoff
(120 – 140 psi), shut off the engine, chock your
wheels (if necessary), release the parking brake
(all vehicles), and the tractor protection valve
(combination vehicle); and fully apply the foot
brake. Hold the foot brake for one minute. Check
the air gauge to see if the air pressure drops more
than three pounds in one minute (single vehicle) or
four pounds in one minute (combination vehicle). If
the air pressure falls more than three psi in one
minute for single vehicles (more than four psi for
combination vehicles), the air loss rate is too much.
Check for air leaks and fix before driving the
vehicle. Otherwise, you could lose your brakes
while driving.

Check Air Compressor Governor Cut-in and
Cut-out Pressures. Pumping by the air
compressor should start at about 100 psi and stop
at about 125 psi. (Check manufacturer's
specifications.) Run the engine at a fast idle. The
air governor should cut-out the air compressor at
about the manufacturer's specified pressure. The
air pressure shown by your gauge(s) will stop
rising. With the engine idling, step on and off the
brake to reduce the air tank pressure. The
compressor should cut-in at about the
manufacturer's specified cut-in pressure. The
pressure should begin to rise.

If the air governor does not work as described
above, it may need to be fixed. A governor that
does not work properly may not keep enough air
pressure for safe driving.

Test Parking Brake. Stop the vehicle, put the
parking brake on, and gently pull against it in a low
gear to test that the parking brake will hold.
Test Service Brakes. Wait for normal air
pressure, release the parking brake, move the
vehicle forward slowly (about five mph), and apply
the brakes firmly using the brake pedal. Note any
vehicle "pulling" to one side, unusual feel, or
delayed stopping action.

This test may show you problems, which you
otherwise wouldn't know about until you needed
the brakes on the road.

Ricelake 922:
Since you live in Peterborough On that means you have or at least should have an Air Brake endorsement. The first cardinal rule you should have learned is if you have any irregularity in an air brake system you take it out of service immediately. You should be more embarrassed not knowing that than asking questions from other people what is wrong. You should also be aware that normal operating pressure is 80 to 135 psi. Since you are driving periodically at 80 you are living and driving on the edge

Possible pedal valve leaking when pedal is applied.....

Do some testing. If single brake pedal push drop the pressure a lot right away - you have too big slack.
When you keep the pedal firmly and the pressure is not building up, or is dropping down, that is indication of leak.
How long the coach keeps the pressure after engine shut down?
I remember my brakes take 3 psi for single activation. Meaning I have to fan them quite a few times for pressure to drop and my alarm comes at 60 psi.

ricelake922 wrote:
Thank you. Will definitely check the loose flair nut.

Maybe, but I think much better odds that you have a leak in an application hose to one, or more, or the brake chambers.

Either way, you need to be able to get under safely and blocked. Then, with full pressure, turn engine off, release the parking brake, and have one hold the brake pedal down with another underneath looking for air leaks. There has to be at least one, or more.

Thank you. Will definitely check the loose flair nut.

Same thing happened to me. Had my BIL press on the brake peddle while is listened. Found a loose flair nut on my left rear drum. Tightened it and all is well now.

To answer your questions....the air dryer and governor was replaced last year. Yes...the air pressure builds and cuts out at 120 psi. Because I am never at a stop sign for long the psi does not fall below 80 psi. The air brakes cut out at 120 and when I am driving it stays at 120.

Couple of problems:

The low pressure alarm should not be sounding at 90 PSI. Most are set to 60 PSI.

That 90 PSI is BELOW the cut-in PSI for your coach. Could be a substantial leak, could be a bad air governor (under $25). How long since the air dryer was last serviced-- that is another source if air problems?

Once above idle RPM does air pressure build to cut-out PSI (probably around 120 PSI)?

If so, how long before it drops to cut-in PSI and starts building again?

There is an air leak somewhere. Does the air only go down when you hold your foot on the brake pedal? Maybe one of the canisters has a bad diaphragm or its an air line.