Parking valve doesn't pop up

it_mike · June 16, 2010, 05:28:05 PM

Failed inspection in part due to the air brake system. Apparently, my parking brake valve is supposed to pop up when the pressure falls below a certain threshold. Any ideas where to start? I'd presume the valve needs serviced or replaced. Additionally there is no 'low air' warning.

Thanks,
Mike

robertglines1 · June 16, 2010, 05:35:22 PM

valve is pre set to pop up at set pressure..replace is easiest about 45$..low air pressure switch is a elect activated switch in air system..ck bulb first..make sure wire is attached to alarm or light...if memory serves me right the low pressure switch is in the spare tire compartment...Bob

bevans6 · June 16, 2010, 05:39:09 PM

what kind of bus? Spring brakes or DD3?

Brian

robertglines1 · June 16, 2010, 05:41:58 PM

Mci 9

bevans6 · June 16, 2010, 06:03:35 PM

My understanding is that the push-pull valve only pops up on low air pressure with spring brake systems. DD-3 is supposed to apply the parking brake automatically at about 40 psi but I don't think the valve pops up. At least it never has on my bus, I guess I will check tomorrow! The low air sensor is mounted on the front wall of the engine bay, on the drivers side beside the rear electrical panel, and reads pressure in the accessory tank. The buzzer in in the front electrical bay. The same buzzer is shared amoung all the alarms through three isolating diodes, so a diode could have failed.

I have to write the air brake exam again (license renewal) in a few weeks so I have to relearn all the spring brake tractor trailer air brake stuff all over again. Which colour glad hand is which, and what a tractor protection valve does and why. Oh well.

If you have a DD-3 system, your inspector needs to know that or it will fail. It's not the same as a spring brake system.

Brian

it_mike · June 17, 2010, 03:42:13 AM

Thanks! I'm sorry, I should have included the bus details.

It's a 1980 TMC Crusader II, which I believe is the same as the MC-9. I can't / don't know how to find the details on the brakes. How do I tell if they're DD or spring brakes?

They also failed me because the tie rod ends seemed too loose on one side and too tight on the other. Is this another unique item that's 'as designed'?

bevans6 · June 17, 2010, 04:14:36 AM

Nope, tie rod ends are simple maintenance items, along with the drag link from the steering box. I remember talking to MCI who said the original tie rod was superceded by a newer version, my bus simply had a whole new tie rod assembly to the new pattern installed when it was brought into Canada, along with a new drag link. They weren't expensive in 2002. If you look at your rear brake chambers and count the hoses, if there are three hoses then they are DD3. Common wisdom has it that it's extremely hard to change the DD3 to spring brake, since the spring brakes are a fair bit larger. It can be done, though, I am told.

I am going to check to see if my push/pull valve pops the same way a spring brake system does this morning. I really don't think it does, though.

Brian

Tom Y · June 17, 2010, 04:32:08 AM

Brian, Yes it should pop up. I think that is what does the work setting the brakes. Tom Y

Tom Y · June 17, 2010, 04:35:22 AM

Mike, The low air alarm senser should be in the spare tire area. The alarm next to the driver in electrical panel? Tom Y

JackConrad · June 17, 2010, 05:29:08 AM

Mike,
You probably have DD3 brakes. To confirm this look at the brake cans on the drive axle. If they have 3 hoses they are DD3, 2 hoses if they are spring brakes. Our parking valve pops up when the air pressure reaches about 40-45 PSI, this sets the DD3 parking/emergency brakes on the drive axle.
Do you have the manuals and wiring schematic? You can jump the wires on the low air sensor to confirm operaqtion of the low air light and buzzer (both are required). Jack

bevans6 · June 17, 2010, 05:58:04 AM

My valve does not pop up automatically, so I guess I have some troubleshooting to do. It's unclear to me if the pressure being discussed is the pressure in the dry tank, or the emergency brake tank. It seems to me that when the pressure difference between the two tanks exceeds 30 to 40 psi, the shuttle valve operates presenting emergency tank pressure to the system.

Brian

Jriddle · June 17, 2010, 06:05:59 AM

Hope you can read this

John

it_mike · June 17, 2010, 06:26:45 AM

Thanks John. I have to get one of these manuals

buswarrior · June 17, 2010, 06:51:53 AM

No matter which system, spring or DD3, the parking control valve will pop up when the air pressure being supplied to it drops below the strength necessary to hold it open against its internal spring.

Typically that's down someplace 25-40 lbs.

In a spring system, the parking brakes already started coming on as the pressure dropped through 60 lbs or so, and you'll be stopped before the parking brake control pops out.

In a DD3, the trouble is, the parking control valve is supplied by the isolated parking tank, which doesn't have a gauge to show you what's going on.

You can't get the parking control to pop on a DD3 without the parking tank pressure dropping far enough.

So, for this "inspection", your inspector needs to do some homework, and get the dust covered DD3 checking procedures out of his library. He was applying the wrong tests to your bus.

happy coaching!
buswarrior

bevans6 · June 17, 2010, 07:11:10 AM

I guess if I kept fanning the brakes, engine off, then eventually I would have lowered the pressure in the parking brake tank enough for the spring to activate the push/pull valve. Another way to check would be to blow the wet tank, the dry tank, the auxiliary tank and then the parking brake tank. At that point the P/P valve should be able to operate. I will go try that out!

My manual copy is handily missing that page, it starts chapter 4 at page 4-3. I always wondered what was on page one and two...

Edit: went and tried it, it all worked exactly as advertised so I thankfully don't have to start troubleshooting anything. Which is handy as I am leaving for a long weekend at the vintage car races at Mosport in an hour!

One more question, sorry to drag this very good conversation out, but I need to know... Page 4-21 of my manual says that the generator control and low air pressure switch is on the front of the left side firewall of the engine compartment, and I know there is a switch there. My manual doesn't have a schematic of the charging system. There is a schematic page that shows the alarms, and that schematic calls out a "FR LOW AIR SW", connected to stud 13 in the front electrical bay. Which is right? My alarm and tell tale light go off absolutely simultaneously with the generator coming on, AND I hear a click of some sort in the front of the bus under the floor or behind the dash. Does anyone know for sure how this bit of the bus is supposed to work?

Brian