Dual air pressure gauge

[Sean]

FWIW, my Neoplan came from the factory with two of these dual-needle gauges.  There is a separate gauge for each brake system, and on each gauge, the red needle shows system (tank) pressure, and the white needle behind it shows application pressure.  When all is well, both red needles read about the same, and both white needles read about the same.  This type of setup lets you know instantly if you are developing a problem in either brake system -- a difference between the gauges either in the tank or application pressures indicates a leak of some kind.  If the difference is in the tank readings, the leak is in the supply system (upstream of the treadle), and if it is in the application readings it is in the service plumbing to the brake cylinders (downstream of the treadle).

Having application gauges like this also lets you become more aware of just how much of your braking potential you are using.  Also, you get a good feel for how much air you actually use each time you apply the brakes.

-Sean
http://OurOdyssey.BlogSpot.com

[buswarrior]

FMVSS 121 came into effect in 1975, requiring, amongst other things, the redundant functionality that is taught in basic air brake courses: isolated dual air system, so that there is some form of braking available under a partial failure. And... the dreaded Anti-skid control.

Then it gets really muddy.

The regulation came into force over time. Some manufacturers received a permit in order to have relief from the full force of the regulations for a limited time, in order to prepare properly. The build plate inside the coach may make reference to a exemption permit.

These were VERY confused times as the government was attempting to regulate when technology was simply not sufficiently developed, and as always, too many decision makers didn't understand the complexity, and mistrusted the politics of the various advocacy groups.

And then the debacle over anti-skid control, where it was regulated, installed, failed, unregulated, disconnected, removed from new vehicles. MCI ordered every fleet owner that had bought one of the anti-skid coaches to disable the units immediately. They simply were unsafe. Coaches wouldn't stop, or they would stop by themselves.

If you have unit 31204 to 31512, it would be interesting to see if the anti-skid control "computers" are still installed in your coach. Let me know, I'd love to have a set for historical posterity.

For the MC8, MCI has air diagrams for:
January 1975, prior to and under exemption from the regulation;
then April 1976, a compliant anti-skid control version with air gauge plumbed to the dry tank;
then December 1976, effectively back to the old way, post debacle;
then April 1978, the air gauge back to the dry tank and the shuttle valve appeared.

In total, IIRC, the MC8 had at least 8 different air schematics, as there were the individual state safety requirements for coaches destined for service in New Jersey and Massachusetts to further complicate matters.

Owners of MCI products (and no doubt the rest!) through the 1970's need to be sure of which variation they have, and how much of the original plumbing is stock, how much has been bypassed, and how it has been accomplished. The guy next door's coach can't be trusted to be the same.

A most confusing decade to be sure!

happy coaching!
buswarrior

[Lin]

My MC5 pressure gauge is only connected to the accessory tank, which leaks down faster than the main system.  The schematics show it that way too.  I guess that the main tank reading would be the most important, in terms of safety, to add on.  The application pressure seems to be of interest, but of less importance.

[justin25taylor]

Sean is correct, as usual.
The reason to have the application pressure is to watch for brake fade while descending a grade.
If you are "applying" more and more pressure to do the same amount of slowing, it is a great indicator of how hot your brakes are getting.
My model 5 Eagle came equipped with that very gauge.

I may be way off here but I always thought that was it's intended purpose.

Best,
Justin

[buswarrior]

yes, you've got it.

happy coaching!
buswarrior

[Lee Bradley]

My Neoplan has dual gauges. One for each system but both needles read that system. No applied pressure reading. Apparently just used to confirm gauge function if the needles don't match its time to check the gauge.

[Oonrahnjay]

(snip)  The last pressure I would want to know about is the accessory tank pressure, sure it's interesting but kind of irrelevant to the safety systems on the bus.   (snip) Brian 

    Brian, auxiliary air is important to me since my transmission is air-controlled.   There is a pressure regulator at 95 PSI off of that tank and the transmission should not be shifted into "go" until the auxiliary tank is up to 95 PSI.  So, for my bus (which is very different in a lot of ways to what most people have), the pressure in the aux tank/system is an important thing to know/keep track of.  So I do have a gauge reading that tank.  My bus stock also ran the doors off the aux system but I've taken the "accordian" doors off and gone to a hinged door so that's not an issue any more.