Add air to bags but not brakes

Scott & Heather · September 10, 2015, 01:48:53 PM

I've searched the forum on this one but must not be entering in the right keywords. I read this somewhere in the past but can't remember. Can I hook my 110v compressor up to our coach to fill the bags for leveling but not the brake chambers? I want my ebrake to stay engaged and not release accidentally if bumped, just like it behaves when no air. But I want my bags to have air for leveling. Where do I connect to do this? I've already added air via the rear chuck in the engine compartment, but I think that allows me to release the ebrake which I don't want...

Oonrahnjay · September 10, 2015, 02:50:29 PM

Quote from: Scott Bennett on September 10, 2015, 01:48:53 PMI've searched the forum on this one but must not be entering in the right keywords. I read this somewhere in the past but can't remember. Can I hook my 110v compressor up to our coach to fill the bags for leveling but not the brake chambers? I want my ebrake to stay engaged and not release accidentally if bumped, just like it behaves when no air. But I want my bags to have air for leveling. Where do I connect to do this? I've already added air via the rear chuck in the engine compartment, but I think that allows me to release the ebrake which I don't want...

Scott, the best answer on this is "it depends". If your system has a suitable one-way valve that's easy to reach (or tubing that's used in the system), then all you need to do is to arrange for an air input into the air suspension system "downstream" of that one-way valve. If not, you may have to modify your system to provide such an input. I can imagine some systems that would be easy (such as taking a plug out of an unused inlet port in a tank and feeding into that) and other system layouts that would be much more complicated.

One the other hand, many systems already provide for a separation between sub-systems. Are you sure that you need to do anything?

RJ · September 10, 2015, 03:04:23 PM

Scott -

Air for the suspension is supplied from the accessory tank, located underneath the driver in the front compartment. It's the last one in line from the engine compressor to get filled, and has a valve in it that will not allow it to fill until the other three tanks have reached at least 60-65 psi.

My 5C's 120vac compressor is tied into the main air system in the engine compartment, so it fills the whole system, just like the engine-driven unit.

I have a really inexpensive solution to your worry about the e-brake releasing and the bus rolling: A set of tire chock blocks from Harbor Freight, roughly $20 for a pair, sometimes on sale for half that. Put them around the drive axle and you're good. Pull them when you're doing your pre-trip walk around (you DO do that, don't you???

KISS principle!

FWIW & HTH. . .

luvrbus · September 10, 2015, 03:11:06 PM

On a D under the drivers window the fill spot there only fills the auxiliary the one in the rear will fill the complete system.Not to be a smart @$# but fill it and push down the park brake valve if it releases the brakes then you know for sure

LuckyChow · September 10, 2015, 07:27:47 PM

I have a 120V air compressor plumbed to my auxillary tank. It doesn not backfeed my brake system.

chessie4905 · September 10, 2015, 07:34:31 PM

You should need more than bump to release brakes. It takes some force to depress release button. Maybe a bump could apply them.

Scott & Heather · September 10, 2015, 09:03:27 PM

Ok. Thanks....I have relocated the compressor from the engine bay to under the drivers area. Will connect to the sabra dee valve there and see what happens. I was pretty sure that schrader valve was for the tow truck to connect to to keep air in the system to release the ebrake but I could be wrong

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bevans6 · September 11, 2015, 04:00:57 AM

I have no idea what a sabra dee valve is. the Schrader valve in the drivers side compartment is the emergency fill, it goes directly to the parking brake tank and the invertion valve and no where else. If you add a fill port to the accessory tank in the driver's compartment (assuming you are talking about your MC-9 here) then you can fill the suspension directly. The pressure protection valve may or may not stop air from going back into the dry tank. On my bus (1980 MCI) the dash pressure gauge reads from the dry tank (brakes, in other words). If when starting the bus and airing up normally the dash pressure gauge starts to build pressure up to around 65 lbs, stops rising for a while, and then starts rising again your pressure gauge is reading the pressure in the dry tank, same as mine. The dry tank fills the parking brake tank directly via a check valve.

Brian

luvrbus · September 11, 2015, 05:20:06 AM

Some 9's are like the C's and D's you will have 2 fill valves under the drivers window 1 for auxiliary fill 1 for brake release

Scott & Heather · September 11, 2015, 06:10:14 AM

Ok so I have one schrader valve under there. And yes is does fill for brake release. Sorry about the mistype. Iphone clumsy thumbs lol. I do have a port I installed years ago to fill that accessory tank. I'll see what happens when I add air there. Brian, I do have the exact same behavior as you in terms of fill, then the pause, then continue to fill while watching the gauge. Tore chocking is of course important, I just would love the added benefit of keeping the ebrake spring locked even while air leveled if it's easily done. And yes, taking about my 9

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bevans6 · September 11, 2015, 06:29:21 AM

I don't know why you think there is an E-brake spring on your bus. Have you changed to spring brakes? The parking brake push-rod lock (which is not a spring in any way with DD-3 brake chambers like you have on your MC-9 as standard) is locked when there is no air pressure present at the locking port on the DD-3. That happens in two situations - when the push-pull valve is operated to engage the parking brake AND there is normal air pressure present in the system, OR when the air system is empty of air pressure (when tanks are drained or simply air pressure has leaked out and the gauge on your dash reads zero). Air pressure in the auxiliary or main system will not affect the parking brake in any way if the push-pull valve in in the parking brake engaged position.

Edit: struck me that I should clarify one thing. If the locking system in the DD3 is activated by the slow loss of air as you might see with a slow air leak, it will lock the push rod wherever the push rod happens to be, in or out. The lock on a DD3 is a one-way lock only, it is designed to keep the push rod from retracting back into the cannister if it is applied, but it does not lock the pushrod from moving further out. If your parking brake is not on when the air leaks out, then in a properly working DD3 type system like in my 1980 generation MC-5C, the push pull valve will pop under it's internal spring pressure and self-activate the parking brake with the residual air pressure in the parking brake tank, which is usually around 35 psi when the valve pops automatically. So in that situation, the pushrod will be out, the brake applied and locked on. It's perfectly possible for a situation to arise where the air pressure leaks out, the push pull valve is broken or otherwise does not operate automatically and for the push rod to be in the fully IN position when the push rod locks due to lack of air pressure at the locking port. In that case, since the push rod lock is a one-way locking sprage clutch, the push rod will be locked into the fully in position and won't be able to go even further in, but it can be pulled out in which case it will be locked as far out as you care to pull it. You do run into this on the bench when you are playing with a dismounted DD3, you pull be push rod out manually and then you can't get it to go back in again until you apply air pressure to the locking port. BTDT, and it is a PITA.

So your goal of having the parking brake remain engaged without regard to system pressure is pretty automatic as long as the push-pull valve is in the on position. Even if you inadvertently moved it to the release position, normally a full brake application is required to release the parking brake. There is a small possibility of the parking brake releasing without a full brake application if the push rod has not moved back the fraction of an inch required to mechanically lock the push rod, but that mechanical lock usually happens very quickly after the parking brake is applied.

Brian

sledhead · September 11, 2015, 08:35:48 AM

what about removing the air bags from the bus system and making your own leveling system ?

that is how my coach is set up . 4 corner leveling that I control with no leaks
they stay at the level I put them at for mths

dave

Jim Eh. · September 11, 2015, 09:17:15 AM

Actually Brian, the parking spring will activate when the air pressure drops. The DD3 has the ability to add additional pressure with a service application and then a lock rod to hold that additional pressure on the brakes for parking.
Better than a regular spring brake ...meh.

bevans6 · September 11, 2015, 09:26:05 AM

Yes, Jim, I was adding that edit probably as you were typing your comment! I worry about getting too detailed because of the several generations of changes in the air brake systems. I know mostly about my 1980 era version, but they changed a lot between then and when they stopped using them, and there were other changes in the early years that I don't know about.

Brian

Scott & Heather · September 12, 2015, 08:18:36 PM

Lol. You guys are thorough!

thanks for sticking with me through this Brian. I need to clarify:

When the bus isn't aired up, I can't push the parking brake knob down without it springing back up. Won't release by design. If I have an electric aux compressor filling my system with air to level, you can at that point push down the parking brake knob and potentially release the brakes. If we happen to be on a little hill, I don't want her to roll if my child accidentally bumps the knob. I know I could put a flap over it etc etc, but I think Dave is kinda thinking along the lines of my question.....isolating bags from brake system or even accessory system so I don't accidentally turn wipers on or push windshield sprayer button etc. just air to the bags for leveling

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