| Author | Message | ||
| Henry 96A3 (Hank)
Rating: N/A |
A few days ago on a recent outing I had to turn the bus around in a tight parking lot. It ended up being like an 8 point turn with a lot of service brake applications in a short amount of time @ low engine RPM's. About half way through this maneuver I ran the system pressure down to about 60psi and had to stop for a while to let it charge back up. It seems to keep up under "normal" driving conditions, like around town and on the hiway. I just had the air dryer rebuilt and didn't notice any difference in the rate of charge. When I start er up for the first time, I run it at slow idle for about 10 minutes. After about 10+ minutes, there's enough system pressure to kick it into fast idle. It takes about five more minutes to bring it up to the 120psi range. When driving on the hiway, it takes about 10-15 min to drop from 120 to 90psi and just a few minutes to charge it back up to 120. When I shut it down it takes about 20 minutes for the pressure to drop from 120 to 60 psi. I spoke with an experienced operator and they told me that the rate of charge was abnormally slow. I plan on checking the system thoroughly for leaks and evaluating the condition of the compressor. Basically, I'm wondering what a "normal" rate of charge is so I know what I'm shooting for. For example, how long should it take to reach full pressure from 0 @ idle? Any thoughts would be appreciated. Henry ('85 96A3) | ||
| Craig (Ceieio)
Rating: N/A |
My MC7 will build enough air for fast idle within one minute. (it will kick in at 30 lbs, but not fully, 45lbs or so has it at max idle). Two to three minutes (with fast idle on) and it will be at 120 lbs and ready to drive off. Not sure what is typical, but that is what I experience. Craig - MC7 Oregon | ||
| Cliff (Floridacracker)
Rating: N/A |
Henry, My GMC 4905 is very close to what Craig is experiencing. I would put on some shop air and listen for leaks with someone else activating the brakes. Cliff | ||
| Ian Giffin (Admin)
Rating: N/A |
Hi Hank, In bus brake systems, we don't usually measure "normal", we refer to benchmarks which, when not met, means a "defective" system. Therefore, typical buildup is not the question - the question is, does the system fail based on the benchmark. Here is the industry guideline: Air Pressure Build-up Time Reduce air pressure in the system to below 80 psi. Run engine at 600 to 900 RPM. Measure the time it takes to build air pressure from 85 to 100 psi. If the air pressure build-up time is greater than two minutes, the braking system is defective. Compressor Governor Settings With the engine running, pump the brake pedal to reduce air pressure. Note governor cut-in pressure. If the cut-in pressure is under 80 psi, the braking system is defective. With the engine running, observe the air pressure gauge. Note the governor cut-out pressure. If the cut out pressure is over 135 psi, the braking system is defective. Low Air Pressure Warning Pump the brake pedal to reduce air pressure. Note the pressure at which the low air pressure warning activates. If the low air pressure warning activates below 55 psi or not at all, the braking system is defective. Air Loss Rate Park on dead level ground and properly chock all wheels. Release parking brakes. Build up system pressure to governor cut-out. Shut off engine. Make and hold a full brake application (pedal to the floor) for 1 minute while observing the air pressure gauges. If, in 1 minute, air pressure drops more than 3 psi, the braking system is defective. If you do any of the above tests and your bus fails, you must not move your bus and you must have your brake system repaired. If you can't do the repairs yourself, you will need to have your bus towed to a repair facility or have a mechanic come to you. Under these conditions, do not drive your bus to a repair facility; it is both unsafe and against the law. Usually, the brakes degrade slowly to the "defective" condition so make the above tests part of your regular daily bus circle check. These are not the only brake tests your bus needs to be put through. There are also tests for the spring brakes, air tanks and a loss rate for an air brake equipped trailer who's air brake system is supplied from the bus through glad hands or similar systems. If you are reading any of the above information for the first time in your life, you should seriously consider taking an air brake course - it may even be mandatory in your state/province. Additional reading should include the "CVSA North American Standard Vehicle Out-of-Service Criteria" manual, as well. Ian www.busnut.com | ||
| David Hartley (Drdave)
Rating: N/A |
So.... What you are saying that is if my air suspension leaks, My brakes are defective? Awww... Come on here !  Some noodle-brain thought that load up for sure! Granted "some" of the tests are valid, The end statement however cannot be validated if there are other sources of air leaks or usage having an effect. The qualification of that would be on a truck whose only air dependency was air pressure systems for braking. A Bus or Coach has several other air dependent systems that can and do affect the outcome of said test standards. I check my air pressures and brakes before every trip even if around the block. Those 50 mph lockdown emergency stop tests work pretty well including find stuff inside that people forgot to tie down or latch like the refrigerator door. Its shocking funny to hear a mayonaise jar tumbling & hurling itself towards the dash! Stuff slides off the shelves and out of cabinets regularly when I test brakes.. | ||
| Craig (Ceieio)
Rating: N/A |
Dave - I first reacted to last item in the checklist as you did, but as I thought about it, I think it is correct. At some level, it does not matter what the source of the air leak is, it effects the braking system, so the braking system is not working right. It just may be that you need to replace a bag or fix an air beam rather than fix a brake system component to fix the issue, but you still have a brake system problem regardless due to air loss out of the system (air supply) that powers the brakes. Craig - MC7 Oregon | ||
| Ian Giffin (Admin)
Rating: N/A |
David, Turn in your Doctorate, laddy! I'm shocked at your response. Shocked!! Ian www.busnut.com | ||
| David Hartley (Drdave)
Rating: N/A |
N.A.R.Phd... Not a real PHD.... Just being vertically literal. Actually running in an 8-point circle. didn't mean to confuse things at all. My comments were Probably caused by familiarity with dual air systems and or primary aux and brake air systems. Which by wrote are supposed to have isolation valves so that air does not bleed back from the brake system primary or secondary tanks into the auxillary air system. The trick to properly assess the situation would be a gauge on the primary brake system feed air line like used in many trucks. This would provide an indication of a brake system fault and not a fault in the auxillary air system as a single combined gauge would indicate. With this gauge then the statements concerning "you have a brake system problem" would be 100% accurate. Some trucks even have a Brake applied pressure gauge along with the feed air gauge. I know, It still doesn't address the compressor capacity but it would be difficult to diagnose without confirming where the air is going. | ||
| Gary Carter Rating: N/A |
The amount of time to rebuild air pressure is also a factor of which engine you have in your bus. Most of the 92s route air from the pressure side of the turbo to the air compressor. Under high boost the recovery time is decreased a lot. | ||
| Henry R. Bergman, Jr. (Henryofcj)
Rating: N/A |
Getting back on topic here, (hint, hint) Henry calling Henry. Does your coach happen to have air assisted power steering which uses air to assist the steering? Maybe, maybe not. But if you doooossss perhaps making all of those lock to lock steering wheel movements just used up all the air because air assist power steering does that. We have the same problem up here in SW Oregon when logging trucks using air assist steering need to spot their trucks within a tight area---they use up all their air quickly not braking... ...but turning the steering wheel a whole lot. Then they have to sit for awhile while rebuilding up air. A real bummer and embarrassing to the driver and stuff like that. Anyway....if you have hydralic power assist or full intergreted power steeering, please ignore my pointless babbling. It's my feeble mind and stuff like that. Crowns forever | ||
| David Hartley (Drdave)
Rating: N/A |
Oh.. My.. Gary, I think that you may want to reconsider what you said... "Most of the 92s route air from the pressure side of the turbo to the air compressor." This is NOT true. Air is routed either from the air cleaner or air cleaner line that feeds the turbo. Good try anyway.. | ||
| Henry 96A3 (Hank)
Rating: N/A |
Dr Dave: I was feeling pretty glum after reading Ian's first post. I figured it was off to truck driving school for me ;-) Of course, my common sense tells me to watch the pressure gauge like a hawk ALWAYS and be on the lookout for excessive air usage-especially during steady service brake applications. If it wasn't maintaining pressure under normal driving conditions, I'd obviously be out and under right now. That spot turning was the only time it ran down and got me worried...that and the slow charge up from the git go. Being new to buses and air brakes I figured this was a good place to get realistic data on 20+ year old vehicles. Henry to Henry: It's a 6V-92TA and it is not air assist steering. Since the air dryer was rebuilt two weeks ago I'll rule that out for now. Air pressure decrease while service breaks are engaged is negligible-until they're released (while engine is running). Pressure is maintained fine in normal driving conditions. I'm going to test it according to Ian's criteria and see how it does. I guess my original question still stands: would it be normal for the air pressure to run down like it did in that particular circumstance-repeated service brake applications at low engine rpms? | ||
| sylverstone (Sylverstone_pd4501864)
Rating: N/A |
pulling my scenicruiser into lukes shop, i had about 80 lbs of air or so (it was still warming up) and the guy pulling it in actually ran the air down enough that the brakes all locked up, just getting it into the shop. (my bus tops out at 105 psi and it doesn't build air fast) *shrug* your brakes can use quite a bit of air if you're on 'em, off 'em, on 'em off 'em repeatedly. if it took you 8 points or so to turn around, and you got it perfect, then you hit the brakes at least 8 times, maybe more, and i know i have a tendency to feather the brake pedal when i'm doing dicey stuff with mine... i was taught "get it done and get off 'em" concerning my airbrakes... but old habits die hard :/ 50 year old bus. 50 year old air brakes. *shrug* i'm lucky it stops at all i suspect -dd | ||
| David Hartley (Drdave)
Rating: N/A |
Yes, Excessive use of service brakes during low speed will reduce the overall pressure available and just about any coach. What you may be seeing is the brake air tank taking more air to equalize from the primary supply which would make the gauge drop faster than normal. If you do a static test by letting the compressor go to cut off and vent (dryer whoosh) and then apply the brakes repeatedly you should be able to see the same effect all the way down to the alarm trip status. Since the compressor is running at a much lower speed at near idle conditions it will build air more slowly than if you were running down the road or if fast idle was in operation while in neutral. You probably have a Bendix or similar compressor like the TU 700 series. These are not the highest volume units and do take a while to build back up from cut-in to cut-off especially when the RPM's are low and air demand is high. The standard rating is: 15.5 cu ft per minute @ 1,250 rpm as taken directly from the MC9 Manual Page 4-7 Prior to that section is information on how the brake system is interconnected to the dry-tank and reserve brake air tank system and the Inversion valve that controls air flow. You may want to look at that as it can affect the amount of air getting to the system. I certainly hope that helps a little.. Dave..... | ||
| John Jewett (Jayjay)
Rating: N/A |
I think he has a blown/leaking head gasket on the compressor. ...JJ | ||
| John MC9 Rating: N/A |
Re: "I guess my original question still stands: would it be normal for the air pressure to run down like it did in that particular circumstance-repeated service brake applications at low engine rpms?" Yeah. It's been awhile since I drove Charters, but yeah, I can remember trying to do a quick six or eight point 180, and having the air drop too low. Actually, that's how an inexperienced driver lost his brakes on the Hunter Mountain downgrade. He was trying to pump the brakes on the fishbowl he was driving on the ice/snow covered road. He ended up running into the Eagle in front of him, and that's what kept him in check for the balance of the downhill ride.. (late 60s) Not to worry, man. The consensus is in! | ||
| Ian Giffin (Admin)
Rating: N/A |
Hank, Really, I didn't make up what I wrote. Consider anything posted in this thread, but understand that we're talking about the life saving braking system on your bus, not a pair of scissors. It would be prudent for you to do some independent research on this topic. I truthfully answered the questions you posed in the last paragraph of your original message. To answer your "original" question, yes, you can easily blow the air out of your system by repeated application of the brake treadle, as you did while turning your bus around. Ian www.busnut.com | ||
| morgan Clough (Mclough777)
Rating: N/A |
Ian, i for one appreciated your comments. i am an ex truck driver and even forgot about the RULES for air. I went out after reading and fired up the bus to check the pressure drop. thanks for all you do and keeping this board up and runnin. i have learned soooo much about buses from here. | ||
| james dean boggs (Jd_boggs)
Rating: N/A |
It's difficult for me to take the air leaks too seriously. Unless it's a gross leak like a stuck valve I once had on the road. Even my maintenace manual says that when replacing hoses to valves etc leakeage is acceptable if you get 2 or 3 soap bubbles size of a dime (at 125 psi) within a one minute. From that I thought that they took that into consideration when they specified the compressor, governor, valves etc. There must be hundreds of air connections on any given bus. Add them all up and you get 20 bucks worth of dimes! So, the compressor has to keep up with the leaks and braking at the same time. I always test my brakes before I go out by stepping on the brake 10 times and check if stays above 90 psi. I can't worry too much about air leaks. I worry about the price of fuel. | ||
| Henry 96A3 (Hank)
Rating: N/A |
Ian, I didn't mean to come across as flippant. I do take the subject seriously and that's why I wrote in the first place. I am new to air brakes I have been studying the manual to grasp the basic layout of the system. I've been reading much material on the subject including our DOT pretrip inspection procedure. My initial question had to do with the rate of charge at idle from 0psi which isn't specified in official literature as far as I've seen and the rate of discharge vs. engine speed in the situation I described. It makes sense that this would vary from vehicle to vehicle and again, that's why I turn to experienced bus operators such as yourself. At some juncture, I too will be an experienced driver so bear with me while I learn the ropes and know that I am very aware of the size, weight and safety implications of driving a 27,000# vehicle...and that I take nothing for granted. Henry | ||
| John MC9 Rating: N/A |
Just aim for something soft, like a Lexus.. | ||
| Henry 96A3 (Hank)
Rating: N/A |
Oh, so THAT'S what was stuck between the dualies! (j/k) | ||
| Buswarrior (Buswarrior)
Rating: N/A |
Hello Henry 96A3 Sorry I took so long to offer this lifeline... I suspect, as does JJ, that your compressor needs work. Your air pressure seems to stay up for a good length of time while driving, so leaks are not the issue here. Try this: build up your air system to cut-out. Pump the brakes down and do Ian's 85 to 100 lb at idle, quarter mile drag race. A good compressor will build that 15 lbs in about 40 seconds at idle. To put this in perspecive, if mine took beyond a minute and a quarter, the compressor is worn or otherwise weak and I'd change it out. The 2 minute standard, though current, is quite old and does not really meet the reliability and performance needed in a coach of our vintages. Dr Dave, how does the new stock cfm rating of the compressor help in this case? If one does not know the volume of air required to fill the coach's tanks? And how does a busnut measure the actual cfm's being delivered? The performance of the compressor at idle is considered of most importance because that is the condition of worst case scenario. For instance, you screwed up the downshift trying to come off the mountain, so now you are out of gear and picking up speed. You are trying to brake repeatedly. Hope the compressor makes some air at idle! Or, you are attempting a multi point turn around... As for arguments of the relative safety of air integrity in different parts of the air system... If the air system is leaking anywhere, the "safety" of the vehicle is compromised, since it is now reliant on various valving to maintain air integrity within the brakes, and with a sufficiently large leak, the compressor's efforts are being wasted overboard. Leaks rarely get better over time, they get worse. Every jurisdiction in North America will put you out of service if you cannot maintain sufficient air integrity to make air compressor governor cut-out. We need to stop this cavalier attitude towards air integrity. Our goal, and our PRIDE, should be toward air tight coaches, not seat of our pants stategies which will clearly not pass the current commercial inspection criteria. Can you imagine shutting down for the night, and being able to fire it up and drive straight away in the morning? An air tight coach is just as possible as a DD that doesn't leak oil. All it takes is the desire and a bit of work. happy coaching! buswarrior | ||
| Stan Rating: N/A |
I have been able to make airtight coaches but I never saw a rusty DD. |