MCI brake question for the Masters... :)

[bevans6]

 My bus is a 1980 spec MC-5C.  The question involves the emergency brake system.  Here is the back-ground:  The emergency brake system is fed by the emergency brake tank, goes to the parking brake chamber, which is 24 square inches compared to the service brake chamber which is 30 square inches.  The air pressure available to the emergency system is limited by a pressure regulator to 85 psi, compared to the service brake air pressure which is up to 120 psi, average around 110 psi.  A full brake applications (IE an emergency stop situation) would have 110 psi times 30 square inches equals 3300 lbs of force from the push-rod.  An emergency brake application would be 24 square inches times 85 psi equals 2040 lbs, or around a 60% application of the service brakes.

Here is the issue - my DOT inspector mechanic thinks the rear drive axle tires should both lock on a packed gravel parking lot with an emergency brake application at 10 - 15 mph.  I think the bus is designed so that can't happen to ensure a safe stop if the emergency brake is suddenly applied, with no locking of tires and loss of control.   I'm not sure even a full service brake application would slide the all tires at that speed.  But - what do you think?  In practice, the passenger side slides a bit, the driver's side does not.  It's an MC-5C so the engine is offset to the driver's side by average 6" or so, so there is more weight on the driver's tires.

Thanks for reading,  Brian

[luvrbus]

Never heard that before logic tell you when you apply the parking valve it takes a few seconds to fill the chambers on the DD3 with air at a reduced pressure and compress the air in the chambers, spring brakes take a few seconds to exhaust  the air to apply also.LOL different strokes for people if a disk brake locks the wheels up then you have a problem ,FWIW spring brakes will apply a lot faster than the DD3 brakes 

[bevans6]

When you said the speed of application, that would definitely affect the lockup potential of a tire.  The emergency brake air has to come out of the emergency tank, go through the pressure regulator (which is definitely a restriction), then go through the inversion valve before it can get to the chamber.  The push pull valve has to activate the inversion valve to let the air flow in the first place.  It's not going to react instantly.

[buswarrior]

That DOT twit has been checking too many empty trucks.

On dry pavement, a disc brake equipped J model might whisper when you pull the knob, but certainly do not lock-up.

Send him back to school.

Parking emergency brakes are rated to hold the vehicle on a slope of xx degrees, found in the regs someplace. AfAIK, they have not added any dynamic performance to FMVSS for parking brakes.

Tossing the parking brake on while moving is a good seat of the pants way to confirm both strength of chamber and slack adjuster setting, but not backed in regulation.

A oil contaminated lining will also stop long.

Better find out what prescribed test methods he is short-cutting on... it'll all be written down somewhere.

Happy coaching!
Buswarrior

[chessie4905]

Get a different inspecter- mechanic. Btw, the friction coefficient of the lining affects stopping ability on any vehicle that has brakes. Some bus companies preferred harder lining that would last longer with some loss of stopping ability.

[bevans6]

A real dearth of DD-3 qualified mechanics here in the wilds of rural Nova Scotia!  Gentleman in question is the go-to heavy equipment mechanic in the area, does all the fire trucks, the DOT vehicles, etc.  He is kind enough to do my bus once a year, and I always have to bring him the manual so he can refresh on the DD3.  I think he is thinking of an empty trailer and dump the air to apply the spring brakes, but he passed me last year, so I don't have any worries.  He does a thorough inspection - two years ago, the regs changed and he had to pull all the brake drums for a complete visual inspection.  That was an expensive visit...

[buswarrior]

Well, an annual tear down of the wheel ends isn't all bad. Won't be any seized fasteners like many busnuts have to contend with!

And it's someone else's back doing the work, so it ain't all bad?

Yup, don't piss off the only game in town!

Happy coaching!
Buswarrior

[bevans6]

Well, that inspection cost me around $400, so that part was bad.  And me being me, I pulled all the wheels and drums the week before just to make sure the fasteners turned and the shoes were dry, the seals sealed, so my back hurt as much as his apprentices did...     Last two years the biggest mileage was driving the 30KM to his shop for the inspection, since travel was interrupted by the great MT-647 installation, which to be honest bought me a lot of respect from him - he was more than impressed with my turbo engine install, but me putting that transmission in was OTT.

Brian