Parking Brake Issue

[fredcliff]

I was asked to start a new thread on this so here I go. We went over and checked the senors, to our surprise they were all good, and after repairing a broken wire everything is fine with the starting. Now the next issue that we are facing. We discovered when we tried to move the bus, when we release the Emergency brake it dumps the air tank.  the air will get up to 120 PSI but as soon as we release the brake it drops to 30 PSI and won't start regaining pressure until the brake is back on   

[eddiepotts]

This is just a guess but have you checked for water in your air tank? you may not have allot of air volume if they are full. how long does it take to rebuild pressure and how much does it loose when brakes are applied.

[bevans6]

What kind of bus do you have again?  different types of parking brakes on different kinds of bus.

edit sorry stupid me, It's your MC9 that you just got to fire!  Late spec DD3, then...

When you release the parking brake on a DD3 system like that, system pressure is applied to the locking port on both rear axle brake cannisters, that is the port closest to the pushrod shaft.  It is what actually releases the locking mechanism on the shaft.  That pressure is supplied from the inversion valve, which is on the rear differential, usually.  If releasing the push/pull valve dumps all the air, and it's dumping at the back as opposed to at the valve at the front, that is the system I would check first.

http://www.bendixvrc.com/itemDisplay.asp?documentID=2393

http://www.buses101.com/PDF_Files/MCI%209%20Maint%20Manual%20in%20PDF%20format/MC-9%20-%20Maint%20Manual%20Section%204.pdf

Another thought - pressure to that system is supplied after the pressure protection valve, which is supposed to operate around 60 psi, not 30 psi.  After you figure this out, I would throw on a new PPV as well.  it's up in the front axle bay on the parking brake tank, I think.  Normal maintenance item that never gets changed.
Brian

[gus]

When I got my 4104 the main/wet tank had been bypassed so cleverly that it was a year before I discovered it.

I had been operating with only the smaller dry tank all that time!

[buswarrior]

Find the big air leak.

Once repaired, let us know if functionality does not return.

happy coaching!
buswarrior