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Lost Air Pressure
So just as I’m about to head up a two lane mountain road to my site in NC ( more about this later) I hear a pop and then a loud hiss and start getting a warning horn and message advising me of low air pressure and to stop the coach. I look down and see my front air reservoir ( tank) is at zero psi , while the rear tank is at optimal 135 psi ( at least at that moment ) I get in a center turn lane and stop the coach to try and get a handle on what happened. With coach running I can hear escaping from somewhere near the front axle. I consult with tech support guy for the converter who built the coach. After some back and forth it’s decided I will limp it into Asheville to a bus service facility where they will get it on a lift to figure out what’s going on, the assumption being the rear tank will be sufficient to operate the braking….bad assumption. While the rear tank wasn’t passively losing air ….each time I had to apply the brakes I lost a little more air pressure , and it wasn’t coming back up. As it got close to 60 psi I reached down and turned the dash air off so I could better hear what was going on. After a while I definitely begin to sense heat at the front of the coach….but no burning brake smell…yet. At this point I knew the plan had failed so I wheeled into a grocery parking lot , not touching the brakes to preserve precious air. I located a sloped area and parked the coach parallel to lift one side for better access. Exiting the coach I expected heat to be coming off the front wheels given the heat I was feeling in the drivers compartment but that wasn’t the case ( only later did I realize that when I reached down to turn off a/c to better hear what was happening I inadvertently turned on the dash heat ) one of the rear drive brakes was definitely dragging however. Anyway after many calls , a friend got a friend to come out and assist ( large wrecker driver, the wrecker not the driver ) while scrounging for wood blocks to block up coach in a friends nearby hot rod shop I spied a 22 Ton bottle jack in a press, we liberated it and back to the coach. Jacked and blocked and discovered the leak was a red emergency air feed line that had not been torqued properly and popped out of the fitting, got it back in fitting and retorqued along with green line next to it that was also loose , presumably from the factory as I just turned 14k miles.
So question: is there a way I could have isolated rear tank ( I’ve been told this is the main and the front is auxiliary or back up) to at least be able to maneuver coach to a safe place? Had this happened on the way up to the resort it would have been a real S__T Show given the narrow roads and switchbacks , with the brakes locked down on a probable steep grade. The rear or main tank was holding air , just not able to make it up because of compromised emergency line. Even tried hooking a compressor to the air chuck at rear of coach to allow us to drive up on a curb for better access with no luck, emergency wouldn’t disengage. Did we miss something , comments ?
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20 Replies
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Your bus is almost new and I assume braking is still the same as our older vintages. You only get one shot from the tank that’s not leaking to get safely stopped and then your done, the spring brakes (emergency) are going to come on at low air pressure. The air leak is so great it cannot be overcome by the compressor, so your dead in the water until the leak can be repaired.
Welcome back the hard way, funny, just the other day I harassed you about never hearing from you on the forum now that you have a new bus like the ole days.
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I know , just when I got the old one figured out , I never learn . What I don’t get is the benefit of the redundancy , if a leak on the emergency side ( or main for that matter ) is going to result in an unwanted application of the brakes what is the point in two systems? Wouldn’t it be better to be able to isolate one or the other to at least have a continuous supply of air ?
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Unfortunately air used to keep the parking brake off is from the primary and secondary systems and the red hose is typically the parking brake air supply line so as pressure is lost the springs apply the parking brake.
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You would think there would be some redundancy, but nope…. I suppose if you capped off the leaking line, you could then build air and keep trucking.
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The alternative is to cap the line, but then the parking brakes have to be caged. Air pressure or caging are the only two ways to get the brakes released.
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If you cap the line and build air pressure, I believe you can release the parking brake. Either front or rear air will release it. it does not take both.
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The issue wasn’t repairing the leak , it was quick access to the problem. In my case the line was in the center of the coach behind the front axle. Even after jacking and blocking the coach it still took some gymnastics to get to it. If there was a cut off at the compressor to the aux air tank I could have been able to build pressure enough to maneuver the coach to a safe place. Having a redundant system that’s not isolated from the other system makes no sense to me, once the integrity of one is compromised they both are affected .
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OK, Gran, now i am confused. The aux system (accessory) is completely isolated from the primary and secondary braking systems and the protection valve allows the brake systems to build to 75 PSI before air flows to the accessory system.
You originally made it sound like the air loss was from an air supply that would drain the primary and secondary systems and not accessory.
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My mistake , the accessory system wasn’t involved ,( i dont think). I have two gauges on the dash which I’ve been told represents primary and emergency ( backup ) the rear tank bled to zero when I lost pressure.
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The brake circuits in your coach are divided into front and rear, just like your car. One tank is for front and one is for the rear. If you cap the line going to that tank the other will build pressure and you can release the parking brake. Most likely it is the supply line has come loose from the tank. Forgot where you are, but if you are at Mtn Falls, or anywhere close, call Midnight Repair. The office has the number. Tom will come to you and fix the leak.
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Is Midnight Repair on the PC map?
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I was able to get it fixed , have used Tom in the past. So a couple of things I’m confused about. There is no “ Main “ and “Back up” air systems , merely front and rear ? I was told the brakes would lock up if not supplied with air , in my case the coach rolled freely until the front tank bled down and the emergency brake deployed and would not reset.
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It is not designed as a backup system air system. It is exactly like the brake system on your car. Front and rear as 2 separate systems with a common operation device (the brake pedal). The purpose of the parking brake is to apply when there is not enough air to apply the brakes. Either the front or rear can release the parking brake. Since that system will have air pressure to operate the brakes. The 2 part or side brake system became a Federal Mandate in the late 60’s.
Unfortunately, it does not operate as you had hoped.
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The rear system is often referred to as the Primary system, as that’s what does most of the braking on a semi. The front system is often referred to as the secondary. The auxiliary system is separate all together on a Prevost, with it’s own compressor but no ability to control the brakes. The front axle does not have parking brakes, so if the front system bleeds down, you won’t apply the parking brake. If the rear system bleeds down, it will apply the parking brake. The compressor can’t send pressure to the rear system if its venting air through the front, so you only have the energy in the tank at the time of the failure. That minimum energy is mandated by FMVSS with tank volume and cut-in pressure requirements. The redundancy is that the rear system didn’t bleed down through your leak due to the check valve and you were still able to stop the bus, not that you can continue driving it with multiple brake applications.
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The rear system will not apply the parking brake if the front has air pressure. Either front or rear will release the parking brake.
This is the reason if both front and rear tanks are leaking down at the same rate check the parking brake circuit as it is common to both.
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