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Old 03-26-2020, 11:11 AM   #1
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PAC brake Question

What exactly is a PAC break as in "how does it work"?

How much should it help -- 20%, 50%???

I have a 2000 Monaco Dynasty with an Allison Transmission.

When I press the PAC break peddle, I can see my transmission meter go from 5th gear to 2nd gear.

But to be honest, I'm not sure I really feel anything. If anything, it feels as if my transmission will "downshift" but definitely not into 2nd gear. It feels like maybe "one" gear change.


What exactly should it feel like to be using the PAC break successfully?

Last time I went down a 6% grade for 4 miles, I had to pump the breaks way more than I would have prefered.

And I am not overloaded by no means.

Thanks in Advance!!!!


Ps...........I've had it looked at; but no one can really tell me if it is working properly. No real "grades" to try it out on.
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Old 03-26-2020, 11:42 AM   #2
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Welcome to iRVs.

Pac Brake , is a brand name of diesel engine " exhaust brake " and these exhaust brakes can be very helpful in controlling downhill speed of diesel engine equipped vehicles.
There are however limitations to their effectiveness .
Engine RPM is the biggest single factor and is why the Allison trans has the downshift program to keep the RPM up . The trans will downshift one gear at a time so as not to over speed the engine and second ( 2) is as low as the program goes.
By closing off almost all exhaust flow , from the exhaust manifold to the muffler , the exhaust brake pressurizes the engine cylinders slowing the engine rotation.
If vehicle speed is too high when the brake is activated , the trans will not downshift even though the exhaust brake flapper has closed off the flow , and the shift indicator shows a low gear.
I prefer to crest any mountain pass at 45 mph, that speed produces 2000 rpm in 4th gear in my coach and the exhaust brake's best performance is between 2000/2500 on my Cat engine.
Remember it's much easier to speed up on a downhill than slow down. So speed at the summit is VERY important.
On long downhills up to 6% grade the exhaust brake will provide the speed control with little application of the service brakes .
On steeper inclines , I watch the tach and when the engine gets to 2500 I apply the service brakes hard to drop the RPM to 2000. If the application of the service brakes becomes too frequent , I apply them to drop the engine RPM below 2000 , at which point the trans program will shift to 3rd. and I again keep RPM between 2000 & 2500 . The engine over speed protection in the trans program will UPSHIFT if I allow RPM over 2750.
These RPM figures are for my Caterpillar engine and you'll have to learn the RPM where your engine, trans program, & exhaust brake work best together.
Safe Travels
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Old 03-26-2020, 12:19 PM   #3
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Welcome to the forum:

This is not much help but someone will be along shortly with great knowledge.
Attached Files
File Type: pdf Exhaust Brake Maintenance.pdf (701.0 KB, 3 views)
File Type: pdf Exhaust Brake.pdf (329.0 KB, 1 views)
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Old 03-26-2020, 12:36 PM   #4
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It might have been glossed over, but do not pump air brakes and do not ride them. A single firm application of the service brakes to get rpm/mph where you want it rather quickly, and then get completely off the service brakes. Your engine brake is primary in any driving above around town, the foot brake (or service brake as it is sometimes called) is secondary. Pumping air brakes will deplete the air faster than the compressor can replenish it, and riding the brakes will heat them up to the point of failure. Coming down the hill, stay in the gear you used going up. Select the PAC brake, use the service brake when necessary in a single firm application of short duration.
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Old 03-26-2020, 01:04 PM   #5
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Yes welcome you'll like it here .

You will scarcely feel the exhaust brake at hiway speed but you should feel it really
kick in at low speeds , when approaching a stop . Until you get used to the brake
try starting down steep grades very slow with the exhaust brake on and
in time you will get a feel for grades . Sometime on a steep grade that I'm
not familiar with I will down shift and go very slow with the semis .
Speed is not your friend , get all that weight moving to fast and it'll be hard to stop

Ray
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Old 03-26-2020, 01:42 PM   #6
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Read the pdf files Myron has linked.

Flapper style exhaust brakes do need periodic lubrication to keep working!
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Old 03-27-2020, 06:27 AM   #7
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Thanks to all!

Great advice gentlemen and thanks for responding.

I have a feeling my pac break simply needs lubrication. The coach is new to me; and the previous owner really did nothing to the coach in terms of maintenance.

Great braking advice as well!

The last Grade I came down (Heading south through Chattanooga to Atlanta) I came over the peak in 2nd gear going about 20 miles an hour; held the PAC break and my coach just would not slow down.

I'm thinking the flapper just did not close. I heard nothing that would indicate the engine was being held back.


Last Question --- what would happen to the braking system if god forbid something broke in the Air Pressure System? Would it feel like it does when automobile's power steering/brakes don't work -- really hard to steer and brake?

I think my RV tech said once that the brakes would lock up...........and I hope he is right. Just wanted to confirm.

I've never looked at those "Escape Roads" going down a steep grade until now!!!

Cheers.
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Old 03-27-2020, 07:34 AM   #8
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On my coach I can tell when the brake engages, it definitely slows the coach but I can feel it more when driving ~45-50 mph. On steeper grades I still have to use my brake some but like recommended I use it by applying a hard constant pressure and the let up.

I do an annual check on my brake. I can access from the back bedroom by pulling the hatch in the closet. I take the linkage off from the small air cylinder and make sure the butterfly valve works freely. I then use the high temp lube that PacBrake recommends, I actually put a couple drops in the air hose also besides the moving parts, this keeps the cylinder lubricated.

From what I understand the park brake is spring apply pressure release. So if an air line broke and you lost enough air the brakes should lock up. Now if you are going highway speed it may not stop you immediately but will eventually bring you to a stop, provided your brakes are in decent condition.

If you have any doubt on your braking or PacBrake working correctly I would have them checked. Hard to stop a +30000 lb coach without good brakes.
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Old 03-27-2020, 10:05 AM   #9
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If your air pressure goes too low your parking brake will kick in basically. The brake system has spring mechanisms to apply the brakes.

This is not your only concern. IF you get those brakes hot they will become useless. Learned that lesson in Death Valley going down a steep hill.

After many many years of driving RV's all over the West with monster hills I thought I knew what I was doing.

One of the previous posters is so spot on. Get slow going down, do not rely on the air brakes. Get in as low of a gear as you need and if you find you are using the brakes much at all get over and let them cool.
Attached Files
File Type: pdf Air Brake general info.pdf (2.64 MB, 1 views)
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Old 03-27-2020, 11:29 AM   #10
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If you develop a leak that the compressor can't keep up with, you will first get a low air alarm. THAT is the time to pull over and stop. The pedal won't feel different, like a car does, it just won't stop as fast.

Once the air pressures drop below 45 psi, the spring brakes are going to apply ( a safety feature ) and stop you, but not nessesserarly where you want to be.

As far as the steering, that's powered by a pump on the engine. In many cases that pump also powers the cooling fan.
If you loose power steering its going to steer like, need I say it, a truck. Very hard to steer but doable if rolling.
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Old 03-27-2020, 01:54 PM   #11
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Thanks to all!

Again................thanks Gentlemen.

The RV community restores my faith in humanity. Cheers.
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Old 03-27-2020, 06:06 PM   #12
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IF you overheat the brakes by pressing and releasing losing air and having he brakes come on by themselves will be useless.

IF your brakes are out of adjustment and the front brakes are not doing their share your rear brakes can fail and again you have a real problem.
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Old 03-27-2020, 06:32 PM   #13
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Quote:
Originally Posted by JeffWestover View Post

Last time I went down a 6% grade for 4 miles, I had to pump the breaks way more than I would have prefered.
Question for you ; What speed are you descending a 6% grade at?
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Old 03-27-2020, 07:09 PM   #14
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Quote:
Originally Posted by JeffWestover View Post
Great advice gentlemen and thanks for responding.

I have a feeling my pac break simply needs lubrication. The coach is new to me; and the previous owner really did nothing to the coach in terms of maintenance.

Great braking advice as well!

The last Grade I came down (Heading south through Chattanooga to Atlanta) I came over the peak in 2nd gear going about 20 miles an hour; held the PAC break and my coach just would not slow down.

I'm thinking the flapper just did not close. I heard nothing that would indicate the engine was being held back.


Last Question --- what would happen to the braking system if god forbid something broke in the Air Pressure System? Would it feel like it does when automobile's power steering/brakes don't work -- really hard to steer and brake?

I think my RV tech said once that the brakes would lock up...........and I hope he is right. Just wanted to confirm.

I've never looked at those "Escape Roads" going down a steep grade until now!!!

Cheers.
Been down that Hill many times with a 10K Race Trailer hanging in Tow.
My Pac brake works Great on that particular downgrade-
My Pac Brake is mounted just after my Turbo and not down stream as most are - I think that really helps with more direct pressure once it's activated - I'm in 4th gear with the brake on all the way down @ 2200 rpm's until I reach the bottom- Tapping the Brake peddle about every 30 seconds or so for about 10 seconds.
Keep it Lubed as it helps- and check for Leaks !!
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