Air system question…how does it work?

[bpu699]

2000 ultimate advantage Winnebago, cat 3126, freightliner chassis.

There are 2 air gauges labeled front and rear…

What does each supply?

Usually, when I am driving or applying the brakes, both gauges decline at the same time, by the same amount. But not always.

For example, driving the tanks fill to 120-125 psi or so. I apply the brakes, both fall by 5 psi or so, simultaneously

If I drive on a smooth road and don’t use the brakes, both tanks stay at max psi for well over 15-30 minutes.

Or, driving down bumpy roads, no brakes applied, usually both fall about the same amount of time after 10-15 minutes, and then replenish.

Is that how it’s supposed to work?

I ask, as part of my last trip, front tanks stayed at 120-125 and only the rear tank lost pressure as I drove over bumps in the road. Front tank stayed at 120, rear fell slowly to 100, then refilled. This happened for about an hour.

Brakes and everything worked fine.

Also, sometimes when the tanks refill they both go to 120-125 or so. Other times when they refill there is a 5 psi discrepancy between the max psi they hit…

And follow up question:

If a front air line to brakes or air ride ruptures, do the rear brakes work indefinitely? Or does all the air pressure get consumed and emergency brake applies?

[bpu699]

Follow up question… nightmare scenario…

Tanks fill to 125 psi, empty to 100psi, then refill.

At 1500rpm that refill takes about 1 second. At idle… longer.

If you reach the top of a big hill and you happen to be at 100 psi at the top, applying the brakes 2,3,4 times can overwhelm the compressors ability to refill…

Thankfully with the exhaust brake, engine rpm usually at 1500-2500 rpm so it refills in a split second.

But if your exhaust brake fails, then what?

Or vice versa. Exhaust brake works, but airline ruptures. Now what? Exhaust brake over 6% grade isn’t slowing you down… you will just speed up, slowly…

What’s the plan of action decending a long decline?

[wolfe10]

Originally Posted by jaydee325

When you press the brake pedal, you are venting air supplying the brake actuators. Springs in the actuators are what is applying the brakes. If you lose air, or air degrades to a certain value, brakes apply. Fail safe.

This is NOT CORRECT!


The service brakes are air applied, spring released.


Yes, on the drive axle, there is also/in addition a second "can" that is spring applied/air released.


Unless you have a massive leak in the air brake system, once you apply the service brake, it does not take any more air to keep it applies (though that is NOT how to safely descend a long/steep grade). Sure, pumping air brakes can use a lot of air, as each application uses air-- NOT the correct way to use them.

[bpu699]

Any thoughts on the way the pressures drop in relation to each tank?

As to pumping the brakes, that’s what the truck driving schools/sites recommend. And every trucker I talked to.

Apply brakes to slow down 10-15 mph, then release. Let speed slightly build back to speed limit, then repeat. Is that incorrect?

Been told applying the drum brakes for more than 5 seconds over heats them quickly.

This only comes into play on long downhills.


Is the service brake enough to stop you on a downhill 6-7% grade???

Slowing on a downhill run, with just air drum brakes, would not be fun… can be done, but much more effective with exhaust brake…

[twinboat]

First, if going down a long grade, downshift to get the engine RPMs up. That will turn your engine and its air compressor faster and replenish your system faster.

If your engine brake is working, it should downshift automaticly, but if it's broken, down shift yourself to pump more air and get down to a real safe speed with one brake application and then keep it under control with the brakes.

When lines break and you loose air, it's usually the front or rear. Use what you've got to stop. If you loose all air, the brakes are going to come on by themselves and should bring you to a stop.

Ignore what Jaydee325 said, he doesn't understand air brakes.

The reason your tank gauge drops on bumpy roads and not smooth roads, is because you have air ride suspension. There are ride height valves that keep the proper gap between the axles and chassis. If its real bumpy or swaying, the valves are adding or releasing air to keep things in the proper position. That air comes from your tank.

[wolfe10]

Originally Posted by bpu699

Apply brakes to slow down 10-15 mph, then release. Let speed slightly build back to speed limit, then repeat. Is that incorrect?

This is correct. This is not the same as "pumping the brakes".


A slightly better way to descend is if you are gaining speed, use the service brakes long enough and hard enough to drop (down arrow) to the next lower gear. Continue until you are in a gear that keeps your speed in equilibrium with no/very little use of service brakes. The equilibrium speed of a diesel pusher on a grade will be faster than a loaded 18 wheeler, slower than an empty one. That is a good judge until you get the hang of it-- if you are passing all the trucks and have to continually use the service brake, you are in too high a gear/going too fast.



And, in all the years on the RV forums, as long as your coach is kept in reasonable condition (read that as keep up with preventive maintenance), this is a non-issue.

[bpu699]

Originally Posted by twinboat

First, if going down a long grade, downshift to get the engine RPMs up. That will turn your engine and its air compressor faster and replenish your system faster.

If your engine brake is working, it should downshift automaticly, but if it's broken, down shift yourself to pump more air and get down to a real safe speed with one brake application and then keep it under control with the brakes.

When lines break and you loose air, it's usually the front or rear. Use what you've got to stop. If you loose all air, the brakes are going to come on by themselves and should bring you to a stop.

Ignore what Jaydee325 said, he doesn't understand air brakes.

The reason your tank gauge drops on bumpy roads and not smooth roads, is because you have air ride suspension. There are ride height valves that keep the proper gap between the axles and chassis. If its real bumpy or swaying, the valves are adding or releasing air to keep things in the proper position. That air comes from your tank.

Hmmmm… if I’m going up hill in 4th at 2000 rpm, and I try to leave it in 4th going downhill, the Allison transmission just up shifts. Without the exhaust brake applied, the motor doesn’t slow the coach down at all… at least I don’t think so…

I do understand that the airbags use up air pressure. My question is whether that should drop pressure just in the rear tank, or both.

Google searches suggest both tanks should have about the same pressure at all times. And, 99.9% of the time mine do. But sometimes, for no obvious reason, the rear tank goes down on bumpy roads, but the front doesn’t. Most of the time, they both deplete equally. Why?

Exhaust brake works great…

[wolfe10]

Originally Posted by bpu699

Hmmmm… if I’m going up hill in 4th at 2000 rpm, and I try to leave it in 4th going downhill, the Allison transmission just up shifts. Without the exhaust brake applied, the motor doesn’t slow the coach down at all… at least I don’t think so…

Yes, the transmission will upshift if engine RPM reaches max no-load RPM (which is usually a little higher than governed engine RPM).


If this happens, use the service brakes long enough and firmly enough to allow a downshift (down arrow selecting 3rd gear) to 3rd where that braking HP can keep your speed in equilibrium.


Also, what exhaust brake do you have-- some require lubrication!

[bpu699]

Originally Posted by wolfe10

This is correct. This is not the same as "pumping the brakes".


A slightly better way to descend is if you are gaining speed, use the service brakes long enough and hard enough to drop (down arrow) to the next lower gear. Continue until you are in a gear that keeps your speed in equilibrium with no/very little use of service brakes. The equilibrium speed of a diesel pusher on a grade will be faster than a loaded 18 wheeler, slower than an empty one. That is a good judge until you get the hang of it-- if you are passing all the trucks and have to continually use the service brake, you are in too high a gear/going too fast.



And, in all the years on the RV forums, as long as your coach is kept in reasonable condition (read that as keep up with preventive maintenance), this is a non-issue.

I’ll try this driving home. I could swear though that the Allison transmission doesn’t really “brake.” Maybe I’m doing it wrong.

If I am going downhill in 4th, exhaust brake off, at 2000 rpm… then the rpm just increases as I accelerate downhill. It doesn’t “hold it.” And as rpm increases, it up shifts.

Maybe I’m wrong, I’ll pay more attention…

[bpu699]

Originally Posted by wolfe10

Yes, the transmission will upshift if engine RPM reaches max no-load RPM (which is usually a little higher than governed engine RPM).


If this happens, use the service brakes long enough and firmly enough to allow a downshift (down arrow selecting 3rd gear) to 3rd where that braking HP can keep your speed in equilibrium.


Also, what exhaust brake do you have-- some require lubrication!

I was curious if the exhaust brake needed maintenance and any failure modes…

[wolfe10]

Originally Posted by bpu699

I was curious if the exhaust brake needed maintenance and any failure modes…

SO, what exhaust brake do you have???? We need that to provide their recommended maintenance.

[wolfe10]

Originally Posted by bpu699

I’ll try this driving home. I could swear though that the Allison transmission doesn’t really “brake.” Maybe I’m doing it wrong.

If I am going downhill in 4th, exhaust brake off, at 2000 rpm… then the rpm just increases as I accelerate downhill. It doesn’t “hold it.” And as rpm increases, it up shifts.

Maybe I’m wrong, I’ll pay more attention…

Descending a grade-- you are correct, transmission alone/no exhaust brake gives VERY little braking. It is the COMBINATION of exhaust brake AND lower gears that WILL hold your speed in check.


Ya, on an 8% grade (very rare) you may be in 2nd gear with the exhaust brake on. But on that grade, a loaded 18 wheeler will likely be under 20 MPH-- you will definitely not be the slowest vehicle on that stretch.

[twinboat]

Engine, actually ehxaust brakes seize up and don't work.

Any engine slowing device can go bad and not work.

Read all about the 121 braking system, you'll see that although split into 2 systems, there are may point where they tie together.

[Iceclimber]

Yikes, so much tension lol.

You can use the exhaust brake AND use the transmission by down arrowing with the panel. When you decend the transmission should say "2" but you are not in 2nd. The automatic transmission is still making the decision. You can override that. I rarely allow a decent to get to that speed, as described I use the air brakes with "brisk application" to keep the speed to my comfort level. Last resort personally would be transmission (I would not hesitate if needed) but to each their own. If you see your air gauge acting funny like dipping while you are driving down the road, you are losing air and it should be fixed. I replaced my entire air dryer and viola everything got much better.
Are you losing are when parked ? If yes, how long does it hold pressure.