air brakes

[Loydd Fones]

    On a 99 Beaver Patriot Thunder with the magnum chassis do the axle brakes dynamite when you lose air pressure or just the driveshaft park brake  ??

Loydd  Fones

[Gerald Farris]

Loydd,
On a 99 Thunder, only the air operated parking brake that is located on the driveshaft is applied when air pressure is lost. The wheel brakes are hydraulic, and therefore do not have a mechanism for applying the brakes when air pressure is lost.

Gerald

[Loydd Fones]

Thanks, that is what I thought I cant imagine why that would be one's emergency system.  I also don't understand why Allison builds a super 6 speed hitech transmission with no park ?

Loydd

[Edward Buker]

Loydd,

You have to remember that what is in your transmission is mechanical gearing in a lubricating fluid, so a brake that is mechanical would involve a pin or a cog that latches. On a 3500lb car that may work very well within the confines of an automatic transmission but our transmissions move upwards of 60,000lbs or more and that kind of load locking would not necessarily be a small somewhat simple locking device. Given the vast majority of the heavy duty vehicles that are mated to this transmission have full air brakes with spring driven parking/emergency brakes there was little need in the design to handle that parking brake issue. While it may be possible, that kind of load parked on a hill with tension on whatever mechanical lock is involved, could be impossible to get to release also.

Later Ed

[Lee Welbanks]

Thanks, that is what I thought I cant imagine why that would be one's emergency system.  I also don't understand why Allison builds a super 6 speed hitech transmission with no park ?

Loydd

If I remember correctly some of the smaller trans had a parking brake, it was a drum and shoe brake added to the rear end of the trans. I can imagine the damage if there was a mechanical gear locking device in one of these 3000 or 4000 trans and somebody was to apply it if the coach was moving as I have seen folks apply their maxi-air parking brakes while still rolling. You would blow the trans all over the ground right there.
With air brakes there is no need for a parking brake.

[Gerald Farris]

Loydd,
The only transmission that Allison builds that incorporates a parking mechanism is the one that GM uses in their pickups.

Gerald    

[Loydd Fones]

  Thanks,  I now understand.   I believe the patriot is air over hydraulic on the axles. but I am not sure

[Gerald Farris]

Loydd,
Yes your Patriot has a hydraulic brake system that uses air pressure as a booster very similar to the way that the vacuum brake booster operates in your car, or otherwise known as power brakes.

To use the hydraulic disc brakes on your rear wheels of your coach as a parking brake, it would be necessary to run cables from the front of the coach to each rear wheel. That would be a very difficult thing to do because of everything between the driver's area and the rear wheels. The cables can be a maintenance issue over time, especially if exposed to road salt. Also, the hydraulic brake calipers that incorporate a parking brake system are very problematic. That is the reason that most cars install a small drum brake system inside the rear brake disc to use as a parking brake. Using the air operated parking brake on the drive shaft is the best alternative for your braking system.

Gerald      

Gerald  

[Loydd Fones]

So if I lose my air  I lose the my wheel brakes ??  I wonder if the the  driveshaft brake would stop me on a downhill path ?

Loydd

[Edward Buker]

Loydd,

In a traditional full air brake system there are spring driven brake chambers in the rear. An air chamber with a diaphragm is used to hold the springs in the off position and if you lose air the brakes apply. In your case you have hydraulic brakes like in a car and you use air to boost the effectiveness of the system. In a typical hydraulic system you can still apply the brakes but the pedal effort will be much greater to get the same braking effect. You may be able to test this by not letting air build up in the system, possibly leaving any air drain valves open, and then see if you held the brake pedal down and put yourself in gear you could test the effectiveness by holding on the brakes and applying a little throttle. You should get a sense of what can be done braking wise without the air boost. A parking lot or a good open area would be a good choice for this type experiment. My assumption is you would still have the parking brake as a back up also during the test.

Later Ed

[Lee Welbanks]

So if I lose my air  I lose the my wheel brakes ??  I wonder if the the  driveshaft brake would stop me on a downhill path ?

Loydd

If while going down a grade or anywhere for that matter and you start to loose air pressure, it is time to park it and find out why. With a true air brake system the spring brakes (Maxi Brake) will auto apply at around 40-50 psig and you will stop like it or not.