Pressure runs too low on MCI-5C

[hogi6123]

When the brakes are pumped down all the way, and you pull and push the yellow button, do you hear air sounds?  I.e. is the parking brake applying and releasing?

[richard5933]

When the brakes are pumped down all the way, and you pull and push the yellow button, do you hear air sounds?  I.e. is the parking brake applying and releasing?

If you're asking me, I can't answer that. If I pump down my brakes the parking brake button will pop out on its own and self apply the brakes as the pressure drops. Nothing to pull since it popped on its own. Pushing it in will do nothing until adequate air is built up to release them.

You don't happen to have a second brake button, do you?

[buswarrior]

Apples, oranges and bananas being discussed...

Vintage of design is mission critical when discussing what "should" be happening.

DD3 parking has a separate isolated tank for its air. Pumping down the service brakes does not lower the pressure in the parking tank, unless the check valve is leaking back...

Unless the system is new enough, for the appropriate regulation to demand it, that's it. There is no "automatic application" as this thread is expecting. The driver has to apply it. Still better than the other period correct vehicles with a hand brake... the driver has something available that can actually get the rig stopped.

Hence the original full name: DD3 Safety Accuator.

Year of manufacture, pre, post FMVSS 121, pre post anti skid control, both the first time and the later time... these things are WAY different in their action, depending...

Happy coaching!
Buswarrior

[hogi6123]

If you're asking me, I can't answer that. If I pump down my brakes the parking brake button will pop out on its own and self apply the brakes as the pressure drops. Nothing to pull since it popped on its own. Pushing it in will do nothing until adequate air is built up to release them.

You don't happen to have a second brake button, do you?

No, it wasn't to you, it was to Chuck:

I pumped it down to nothing on the dash gauge, no further air sounds or resistance on the pedal, and the parking brake knob didn't pop out.  I'm referring to the yellow push/pull parking brake knob.  It's pretty obvious if it's engaged or not.  That's what you're referring to, correct?

When the brakes are pumped down all the way, and you pull and push the yellow button, do you hear air sounds?  I.e. is the parking brake applying and releasing?

[richard5933]

Apples, oranges and bananas being discussed...

Vintage of design is mission critical when discussing what "should" be happening.

DD3 parking has a separate isolated tank for its air. Pumping down the service brakes does not lower the pressure in the parking tank, unless the check valve is leaking back...

Unless the system is new enough, for the appropriate regulation to demand it, that's it. There is no "automatic application" as this thread is expecting. The driver has to apply it. Still better than the other period correct vehicles with a hand brake... the driver has something available that can actually get the rig stopped.

Hence the original full name: DD3 Safety Accuator.

Year of manufacture, pre, post FMVSS 121, pre post anti skid control, both the first time and the later time... these things are WAY different in their action, depending...

Happy coaching!
Buswarrior

Doesn't make sense what you're saying, at least not to me.

My manual clearly states that when pressure in the system drops to below 40 psi the brakes will automatically apply. And they do - when I do a pump down test during the pre-trip the button pops out every time at about 35-40 psi.

The manual for my bus, which came from the factory with DD3 brakes, specifically says that if the driver does not apply the brakes manually when there is a malfunction they will apply automatically.

Are you saying that the MCI doesn't have this safety feature?

The D D 3 stands for Double Diaphragm 3-Triple Action (service brakes, parking brake, emergency brake.)

[Ed Hackenbruch]

The brakes on my 68 MCI 5A would apply when the pressure would drop down past a certain point.

[hogi6123]

Doesn't make sense what you're saying, at least not to me.

My manual clearly states that when pressure in the system drops to below 40 psi the brakes will automatically apply. And they do - when I do a pump down test during the pre-trip the button pops out every time at about 35-40 psi.

The manual for my bus, which came from the factory with DD3 brakes, specifically says that if the driver does not apply the brakes manually when there is a malfunction they will apply automatically.

Are you saying that the MCI doesn't have this safety feature?

The D D 3 stands for Double Diaphragm 3-Triple Action (service brakes, parking brake, emergency brake.)

My bus, 1981 MCI MC-9, with DD3 brakes, manual says:
"Parking and emergency brakes will not apply automatically until parking reservoir pressure drops below 40 psi."
"WARNING: Air pressure gauge on the dash does not show parking brake reservoir pressure"
"When a loss of air is experienced in the service brake system, ... the shuttle valve ... allows air pressure from the parking tank ... to apply the rear brakes."

My bus will automatically apply the parking brake, but the dash gauge cannot be used to know when the parking brake should apply automatically.

I have to pump the brake until the air is exhausted from the main system and then continue pumping the brake to exhaust the air from the parking tank before the parking brakes finally apply on their own (and the knob pops up).

Obviously a manual is necessary here to determine the intended behavior.  Then the owner can determine whether any valves are malfunctioning.

[richard5933]

My bus, 1981 MCI MC-9, with DD3 brakes, manual says:
"Parking and emergency brakes will not apply automatically until parking reservoir pressure drops below 40 psi."
"WARNING: Air pressure gauge on the dash does not show parking brake reservoir pressure"
"When a loss of air is experienced in the service brake system, ... the shuttle valve ... allows air pressure from the parking tank ... to apply the rear brakes."

My bus will automatically apply the parking brake, but the dash gauge cannot be used to know when the parking brake should apply automatically.

I have to pump the brake until the air is exhausted from the main system and then continue pumping the brake to exhaust the air from the parking tank before the parking brakes finally apply on their own (and the knob pops up).

Obviously a manual is necessary here to determine the intended behavior.  Then the owner can determine whether any valves are malfunctioning.

I suppose it doesn't matter as much what the gauge says, as long as the brakes do automatically apply when appropriate.

[buswarrior]

There is no "shuttle valve" in earlier coaches.

How these work is different, depending on the year of manufacture.

How these work is wildly different, if the various parts have not been proven to be functioning correctly.

What one busnut's bus does, is no indicator of what another's bus will do.

Just another round of incomplete information...

Brian Evans and I have typed out tons of stuff over the decades, both here and over on BNO.

No immediate idea where all my books are hiding, haven't needed them for many years.

Use the search and much better typing than either of us can muster these days, will appear?

Happy coaching!
Buswarrior

[hogi6123]

How different is a MC-5C made in 1979 from a MC-9 made in 1981?  I don't know, and probably never will.  I'm pretty sure most owners know only their own bus.  I'm trying to say "this is how my bus works, yours might be similar".  Did I present the information the wrong way, or should I have just stayed silent?  I try to be helpful but sometimes I'm not sure if I am being helpful or not.

[richard5933]

There is no "shuttle valve" in earlier coaches.

How these work is different, depending on the year of manufacture.

How these work is wildly different, if the various parts have not been proven to be functioning correctly....

Still somewhat too cryptic for me...

Even if my 1974 bus doesn't have a shuttle valve, it clearly applies the brakes and pops the button when pressure drops. Manual says it does and the pre-trip pump-down test confirms it.

Are you saying that some buses with DD3 brakes will not do this?

[buswarrior]

Are you saying that some buses with DD3 brakes will not do this?

Correct. Earlier, the brake was there, charged and ready to apply, just needed the driver to pull the plunger.

Combination of safety evolution, and regulation, that the machine was designed to stop itself, if the service air went away, since the driver at that point was proven to too often be terrified, frozen, to take further action.

Happy coaching!
Buswarrior

[chuckdrum]

Gents,

Thanks for the various inputs here.  When you see things from enough angles, eventually you can figure it out!

Attached are a couple pics that add some more evidence to the investigation. 

-the pics make me think I again need to consider compressor replacement.  FYI, according to the PO's notes, the compressor was rebuilt in 2010; probably has about 60K+ miles on it since then.  I've seen "ya never know what you're gonna get" comments on rebuilt compressors in various discussions.

-Still leaning toward the governor being at least one of the problems

-not sure about the parking/emergency brake valve based on the past few posts (will look further but can't find a definitive answer in my manual) My gut and experience with this coach and others I used to own (back in my youth) suggests it should be automatically engaging at low pressures.

In answer to previous questions:
Richard- the aux compressor was not on while I was doing the brake tests, but it's plumbed into the system so was part of it, judging from the aux gauge. 

Hogi6123-  I'll have to do the zero-pressure check again, as I don't recall any air sounds when I engaged it at no pressure on the dash gauge; it was very slight if there was anything, but I'll check it again.

[richard5933]

Don't know if that oil leak is indicative of anything other than a bad/leaking fitting and/or hose. It's when the oil is getting into the air that there's a problem. When you drain the muffler petcock or drain the tank, do you get anything besides air and water? Does the water look like it has oil in it?

[RJ]

Gents,

Thanks for the various inputs here.  When you see things from enough angles, eventually you can figure it out!

Chuck -

Just an FYI, the stock air compressor and governor are most easily accessed thru the floor hatch inside on your 5C, rather than thru the curbside engine door

If you end up needing to replace the air compressor, note that because of the angle it's mounted at on a 5C, it's not a model that's typically on the truck parts house shelf. Luke's your best bet for the right part the first time. 1-888-262-2434

The governor, however, is a stock item.

Oh, be sure to drain the "ping tank" that's on the back of the wheel well near the top inside the curbside door. Should have minimum to no gunk at all coming out when you open the drain petcock.

Sounds like you've got quite a bit of delayed PM to do!

HTH. . .