Seems to me the OP was trying to educate himself here.  He's less likely to
continue to do so after belligerent posts like yours.

Sheeesh!!!
Plunk

LOL. (Make up your own joke here.)

GB in NC

What rinky dink outfit do you drive for?  I want to make sure to never be
where you are.  You don't know sh.t about air brakes and you're going to
kill somebody giving them erroneous advice.  Show me a reference that
tells how you think air brakes work, and I'll show you 100 that say you
are wrong.

Here are four references that say you are wrong.  All say that air
pressure is applied to the service brakes to apply braking power.  All say
that the low pressure warning is 60 psi.  Some buses may be higher at 80
psi.  

None say the brakes apply at less than 20 psi.  None say that air pressure
is removed to apply service braking.  

I await your apology, though I won't hold my breath.  My money is that you
will slink off into the nether without relying because you know you are
wrong, and I have proved it.

http://www.e-z.net/~ts/ts/brakpg.htm
http://www.dot.state.ia.us/mvd/ods/cdl/section5.pdf
http://www.dmv.ca.gov/pubs/cdl_htm/sec5_a.htm
http://209.85.165.104/search?q=cache:iHDdCTW3upcJ:www.nydmv.state.ny.us/broch/cd
l/cdl10sec05.pdf+air+brakes&hl=en&gl=us&ct=clnk&cd=16

Bust driver?   Count me in!

BTW I am a bust driver and I know of what I speak.

Frank is the one that gave the correct information, and no busses and motor
homes use exactly the same system, the only difference would be in the type
of shoes, or what was used in earlier systems of air over hydraulic. However
in both systems lack of air will activate the spring brake and the air will
be required to apply the service brake, and both systems require min 60 psi
before the springs are sufficiently compressed to release parking brake.
Rudy

You definitely are a "bust" driver.  And the fallacy you believe about
air brakes is a common one but - just like liberal ideas - being
commonly held doesn't make it true.

Pay attention and learn something.  Or don't - see how much I care.

This is cut from the article I wrote for the Dec. issue of Bus
Conversions.  An article written, BTW, at the editor's request to
correct the fallacy dipshit Dave has just perpetuated.

Despite what many truckers believe, air brake systems operate in a
very similar manner to hydraulic brake systems.  If you strip away all
the complicated air circuits, relay valves, dual systems, etc. you are
left with a foot treadle that applies air pressure to the brake shoes.
When the brake shoes contact the drum heat is generated which
dissipates the energy of the moving vehicle and ultimately stops it.
Obviously it is more complicated than that, but the principle is the
same as stepping on the brake in your car which transfers pressure to
the brakes through hydraulic brake fluid.  

The reason confusion arises is because older brake systems - single
chamber systems - were exclusively air operated while newer systems -
dual chamber systems - also involve springs.  The important point to
remember is that the springs are for parking - nothing else.  When you
step on the service brake the only thing that happens is that air
pressure is applied to a piston which acts through a push rod, slack
adjuster and S-cam to apply your brakes.  

A single chamber air pot houses a diaphragm attached to a pushrod.
When air pressure is applied to the pot the pushrod extends.  To apply
parking brakes with this type of brake pot it is necessary for the
system to hold air pressure indefinitely.  Obviously that doesn't
happen and that is the reason that spring brakes or dual chamber air
pots were developed.  

On a dual chamber air pot there are - surprisingly enough - two
chambers.  One chamber houses a great big spring and a diaphragm.  The
other chamber houses a diaphragm.  Either the spring or the diaphragm
or both have the ability to extend the pushrod.  On a dual chamber or
spring system when you push the brake release button you apply air to
the rear chamber on the pot.  The diaphragm in that rear chamber
compresses the spring.  At that point the spring is not extending the
push rod so the brakes are released.  When you pull the knob to apply
the brakes the whoooossssh of air you hear is the air leaving the rear
chamber, releasing the spring which extends the pushrod and applies
the brakes.  

When you step on the brake treadle in a dual chamber system you apply
air to the front chamber which also extends the push rod and applies
the brakes.  The term "compounding the brakes" refers to the situation
where a newbie driver applies the service brakes at the same time as
the parking brakes are being applied by the brake spring.  Newer
systems have anti-compounding circuitry but it still is a good idea
not to use the service brakes while the spring brakes are applied.  

The only other item worth noting in this discussion is what happens if
you lose air pressure on a spring brake system.  At some point between
80 and 60 PSI the springs will start to overcome the air pressure that
is caging them in the rear chamber and your brakes will start to drag.
Eventually the application valve will pop which will fully apply the
spring brakes.