Tag Axle Air Bags.

[Iver]

Just moving the coach(MC-9) around the yard and I noticed the left tag axle tire locking up and skidding at moderate braking pressure.  When I adjusted the brakes last I tried to keep the tag brakes slightly less sensitive than the duals. 

It doesn't look like there is much weight on that tag so my question is how can you tell if the tag air bags have the right amount of pressure?  It's pretty obvious to tell when the other bags are low, but the tag bags just hang there extended.

The previous owner installed manual levelling valves and all the gauges show approx. 60 lbs.  Does the air supply for the drive axle air bags also supply air to the tag bags?

Should I try to jack up the tag a little to see if the bag collapses?
        Thanks,  Iver.

[JackConrad]

Iver,
   Per specs, the tag air bags are supposed to carry 35 PSI (there is a regulator on the bulkhead in the engine compartment).  You can follow the air lines from the tag axles to the control valves inside the passenger side engine compartment door (if so equipped) and to the regulator. Some of the buses had a electrical switch on the panel left of the driver to release the air pressure in the tag axle air bags.  Look at the air schematic in your manual. 
    With the bus aired up, push on the tag axle air bags. both should have the same firmness. Spray water around the top & bottom of the air bag to check for leaks. Another possibility is that the tax axle is stuck in the crossover tube. If the bus has sat for a while, the air suspension leaks down and the bus settles. When you air it up, the bus raises, but the tag axle does not move the full distance putting less pressure on that wheel. Note: tag axles are independent suspension.   Jack

[buswarrior]

Tag axle brakes are the first ones to hang up due to rust, as they have the least weight on them. The rest of your brakes are probably in the same condition, they just can't show it.

As well as the checks for proper tag axle movement and inflation, you want to do some heavy brake application, fore and aft running on pavement and get the rust off the drums.

happy coaching!
buswarrior

[JackConrad]

     When we first got our MC-8, we had a problem with the tag axles locking up on hard brake applications (like when the idiot pulls out right in front of you). After installing a brake application gauge, I noticed they never locked up at less than about 30-35 PSI application pressure.  One day, while looking up something unrelated in the parts manual, I found a listing for a "Tax Axle Brake Regulator" that was preset at 25-30 PSI.  This part was used starting with a serial number that was built shortly AFTER our bus.  I went to Northern tool and purchased 2 adjustable regulator and set them at 27 PSI.
  After installing the regulators, i decided to do a test drive.  At about 50 MPH, I put the transmission in neutral and applied the brakes as hard as I could (about 65-70 PSI brake application pressure. Not a sign of tire smoke YES, this is good. I put the transmission back in gear and applied throttle, bus would not budge, NOT GOOD!.  I looked down and brake pedal was still fully depressed, REALLY NOT GOOD. I tried lifting it up with my foot, wouldn't budge.  I got up, walked outside the bus and looked in the compartment under the driver's seat at the brake linkage, nothing bent or out of place. When I got back in the bus I noticed a short stubby screwdriver wedged under the bottom of the brake pedal. I removed it and the pedal returned to normal position.  BIG SIGH OF RELIEF.  Apparently I had left that screwdriver on the floor under the driver's seat and with the super hard brake application it landed in the wrong spot.  Jack

[Iver]

Great story Jack,
   I love it when that happens.....not the problem part but the part when a seemingly big problem turns out to be nothing at all and you get the "BIG SIGH OF RELIEF".....
I have been reading up in the book about the tag air bags and I located the air regulators up on the bulkhead and traced all the air lines. One regulator for 35lbs and the other for 15lbs? I think.  When the coach was aired up the bags seemed fully stretched out.  After sitting for a day or so, the bags have collapsed somewhat as the air in the system has been bleeding out.  So it looks as though they are filling okay.
   My system has the switch on the driver's panel to lower the pressure in the tags so there are no valves just inside the side engine door.  But I can see where there are tee fittings in each line where it would be possible to place a gauge. That way I could monitor the pressure in the bags to see if the regulators are adjusted correctly.
   As I said, the previous owner installed a levelling system and there are gauges on the dash that monitor and adjust the 4 corners and the convoluted bags.
Also, I'll pull the coach over my pit again and recheck the brake adjustment on the tags.  But first I'll take it for a spin as buswarrior suggested.
           Iver.

[JackConrad]

Iver,
  I have a few MCI owners tell me that they increased the pressure in their tags to 40 PSI and saw an improvement in handling.  We have not done this, so I can't comment from 1st hand experience.
   You mentioned that you have a leveling system on your coach.  When we installed our leveling system, I added solenoids on the tag axle airbags to allow me to dump the air in them. I did this so that is leveling for camping required the rear of the bus to be lowered, dumping the air in the drive axle air bags would not throw all that weight onto the tag axle air bags.  Jack

[Iver]

Hi Jack,
   Yeah, I was thinking the same thing.  If I level the coach by deflating the rear bags I should unload the tags first.
I was talking to an old 'busnut' today and he mentioned that many coach line mechanics backed off the brakes on the tags to keep the driver from skidding the tag tires and creating flat spots which would thump and bump causing the passengers to complain. Obviously backing off the brakes would be not good for an inspector either.
   I like your idea of regulators.
          Iver.