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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.
It doesn’t lose air. Rather, sometimes the tanks are in pressure synchrony and sometimes not. No one has really commented on whether that’s normal or why it occurs…
If the airbags are off the rear tank only, then driving along, why doesn’t just the rear tank lose air? Most of the time, both tanks lose air in synchrony going over bumps. But not always.
It doesn’t lose air. Rather, sometimes the tanks are in pressure synchrony and sometimes not. No one has really commented on whether that’s normal or why it occurs…
If the airbags are off the rear tank only, then driving along, why doesn’t just the rear tank lose air? Most of the time, both tanks lose air in synchrony going over bumps. But not always.
To clarify, please. Are you saying if the coach is parked for a week - the air pressure stays exactly the same with no loss ? That is somewhat rare.
The tanks are protected so one part of the system, like say the air bag side - can't lose all its air and cause the air brakes to fail. That's why. Generally speaking the tanks should equalize but one side acting different is a hint. If you are "losing air going over bumps" then either the gauges are wonky or you are experiencing a leak. My guess is air dryer purge valve, but I am a mere novice compare to many here. If you are really worried, have your air system inspected at a truck service place.
Here's an air system diagram for my 99 chassis .
Air suspension takes air from the front tank , as does the exhaust brake , air horn and customer air fitting .
Your exhaust brake could possibly be a Jacobs product . Picture of the ID tag on my 3126B.
I drive downhill by the tach , with the exhaust brake on , and 4th gear selected at the summit , I watch the tach and at 2500 RPM , I apply the service brakes hard to drop the revs to 2000 , then release and coast with the exhaust brake doing the speed control .
Above 2500 RPM . the trans programing could upshift for engine protection .
Below 2000 RMP ; the transmission will downshift to the next lower gear and RPM will go up accordingly.
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99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.
Pac Brake markets a special lube for exhaust brakes .
This diagram for lube points is from Pac Brake and because the Jacobs brake has an exposed shaft end ( yellow box ) it should be sprayed with lube too .
Points , A,C,D,and E should be sprayed.
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99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.
Pac Brake markets a special lube for exhaust brakes .
This diagram for lube points is from Pac Brake and because the Jacobs brake has an exposed shaft end ( yellow box ) it should be sprayed with lube too .
It doesn’t lose air. Rather, sometimes the tanks are in pressure synchrony and sometimes not. No one has really commented on whether that’s normal or why it occurs…
If the airbags are off the rear tank only, then driving along, why doesn’t just the rear tank lose air? Most of the time, both tanks lose air in synchrony going over bumps. But not always.
Funn / sad that no one actually answered the real question…. I will give it a try.
Different chassis have somewhat different air tank arrangements and labeling. Some may label the tanks front and rear, some may label them “1” and “2”. In either case, one can be thought of as “primary” … it supplies air to the brakes. The second tank (“rear” or “2”) supplies air to the suspension (IIRC). They are used for two separate purposes, and so it is quite normal for them to fall at different rates. Regularly on my rig, tank 1 might stay at 130 or so while tank 2 falls to 100. Also, IIRC, tank 2 acts as a “backup” to tank 1, so that if the pressure in tank 1 falls below 90 or so, a check valve releases pressure from tank 2 into tank 1, giving you some time to stop and address whatever caused the pressure loss. Bottom line … sounds like your system is running just fine.
As to the other discussions causing tension … with good practice I would simply not worry about your “nightmare scenarios”. Forget the “ go down in the same gear you went up” dogma. Before going down, pull over and do a brake test. Then start slow and find that “equilibrium” speed iceclimber mentions (using just your gearing and very occasional jabs at the brake). If done right, you will never get going fast enough that a combination of downshifting/ braking / parking brake application - as a fail safe - and or engine brake will not bring you to a safe stop.
A couple years ago we headed west down wolf creek pass, and my wife got nervous after hearing about the hairpin curve. Going down I hit that equilibrium speed at 20-25 mph … slow, steady .. quite relaxing … and I never touched my brakes once. I think people get into trouble if they feel they need to keep their speed up. Instead, let the rig tell you what speed it wants to go at. Just get out there and practice and you will get the feel for it.
You are driving down the highway, periodically applying the brakes. Your air gauges are more like scenario 1, or 2?
You are driving down the highway, not using the brakes, but air suspension undulating due to bumps and using air. Your gauges look like scenarios 1, or 2?
Mine have always looked like scenario #1, but on rare occasion, look like scenario #2. Under the exact same driving conditions. Why?
You are driving down the highway, periodically applying the brakes. Your air gauges are more like scenario 1, or 2?
You are driving down the highway, not using the brakes, but air suspension undulating due to bumps and using air. Your gauges look like scenarios 1, or 2?
Mine have always looked like scenario #1, but on rare occasion, look like scenario #2. Under the exact same driving conditions. Why?
Generally scenario 1, but scenario 2 isn't uncommon either. Other systems like the pacbrake or air suspension, air horns, etc. use air from one tank vs another, and could account for the difference. In addition, even braking lightly might not fully engage all front/rear brakes, introducing a difference in their respective air pressures. Finally, you could have a leaking exhaust valve on one system vs another that only occurs sporadically.
Your Freightliner air gauges are electronic , not manual , so there is a sensor involved , in the VDC ( Vehicle Data Computer ) and there a known issues with the sensor connections in the VDC .
Also if you have no service info on your air system dryer , bringing it's service up to date would be a very good idea.
I've never spent the kind of time watching the air gauges to the extent that you have so , I'll pass on commenting on pressure drop in the situations you've described .
Your air system's low air warning is set for 65/70 psi and park brake auto apply 45 psi .
Here's some reading material that I've collected on our vintage Freightliner chassis .
EDIT: It would be interesting to know if your home state requires a non commercial drivers license and related air brake training for you to legally operate your coach .
Such training would have answered just about all your questions .
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99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.
Your Freightliner air gauges are electronic , not manual , so there is a sensor involved , in the VDC ( Vehicle Data Computer ) and there a known issues with the sensor connections in the VDC .
Also if you have no service info on your air system dryer , bringing it's service up to date would be a very good idea.
I've never spent the kind of time watching the air gauges to the extent that you have so , I'll pass on commenting on pressure drop in the situations you've described .
Your air system's low air warning is set for 65/70 psi and park brake auto apply 45 psi .
Here's some reading material that I've collected on our vintage Freightliner chassis .
EDIT: It would be interesting to know if your home state requires a non commercial drivers license and related air brake training for you to legally operate your coach .
Such training would have answered just about all your questions .
Thanks for the info, do appreciate. My state doesn’t require any air system test knowledge or testing. That said, I did read other states requirements and passed sample tests. I think I under stand the system to a degree. I’ve also looked at plumbing diagrams.
The issue regarding whether psi should match between tanks isn’t in any training info I’ve seen. But several sites state the tanks should always be within 5 psi of each other…
Most truck sites state that the front and rear air systems function independently, creating an extra measure of safety and redundancy. I’m trying to understand how that’s so. If both tanks loose pressure equally, then by definition how are they “independent”? I would think if they were independent, then with a ruptured line/air bag, one tank falls to 60. Other stays at 125.
When I blew an airline, both tanks were loosing pressure from what I recall.
I know I’m overthinking it, but that’s in my dna..
The issue regarding whether psi should match between tanks isn’t in any training info I’ve seen.
As I mentioned , having run my operational tests on the low air alarm and the park ( spring ) brake function at low pressure , I tend to rely on the automated systems rather than constantly watching the gauges for slight discrepancies .
I do check air pressures at highway summits , and if I ever found pressures low after a climb would stop immediately, in the truck brake check area.
In 13 years , that's never happened.
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99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.
Funn / sad that no one actually answered the real question…. I will give it a try.
In either case, one can be thought of as “primary” … it supplies air to the brakes. The second tank (“rear” or “2”) supplies air to the suspension (IIRC).
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Your statement above Is not how it works.
The primary tank supplies the rear brakes and the secondary tank supplies the front brakes. This is done so that there will some sort of brakes even if one tank is ripped off the vehicle.
Suspension could be feed of one tank or the other.
There is also a 3rd tank in the system, called the wet tank. No gauge and recieves all of the air from the compressor, thru the air dryer. From that wet tank the system is split to the 2 gauged tanks with double check valve tees.
Linear Mode
