Air brake test and Parking brake button.

[was68]

Hello Forum people.  I own a 1991 102c3 and would like to know if when doing an air brake test is the emergency brake supposed to pop out at 25-40psi? All the break tests online show yellow dash buttons popping out, My button is next to the driver seat and under a spring loaded cover. My friend said that if I ever get pulled over and asked to do an air brake test that this button not popping out would allow them to remove me from the road. Please help me out.

Wade Scribner
Vancouver, Wa

[Hobie]

Yes.  When it does, the spring brake is applied.

[lostagain]

Wade, pump the brake pedal to exhaust air out of the system. At around 80 psi, the low air light and buzzer will come on. Then at 20 to 40 psi, the parking/emergency brakes will come on, and the push/pull valve beside the seat will pop up. That is part of the commercial vehicle daily pre trip inspection. Does your bus have DD3 brake chambers, like your MC7?

JC

[Boomer]

With DD-3's the button will not pop at normal spring brake pressure, and in fact, sometimes not at all.  That is the nature of DD-3's.  We trained our drivers to mention this to DOT inspectors prior to the pump down because few of them were trained on the DD3 system.  What you mention is normal (unless you have spring brakes).

[lostagain]

I should have clarified: DD3 chambers will come on with loss of air, but the push/pull valve will not pop out. With spring brakes, the valve will pop out. Is the C3 with DD3s or spring brakes?

JC

[bevans6]

With the MCI DD3 system the push pull valve is supplied with air from the emergency tank, not the service tank.  The gauge reads the pressure in the service tank and the emergency tank is separated from the service tank with a check valve so it stays at full pressure even when the service tank is at zero.  With this in mind, the push pull valve will indeed pop at 30 - 40 psi when the pressure in the emergency tank falls to that level.  Since there is no gauge on that tank, it's very hard to tell what the pressure is when that happens.  If you want to test this, fill the air system to 120 psi, turn the engine off, drain the wet tank, then the auxilliary tank, then the service tank all to zero, then drain the emergency tank.  The first test is the check valve - if the check valve is working the emergency tank will still have full pressure after all the other tanks are at zero.  Drain it down, and the push pull valve should pop when it gets somewhat close to empty - the sound from the drain valve changes and the air is coming out slower.

Spring brakes are different.  They start to come on at 60 psi.

Brian