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Pneumatic System Checks
On another forum is a discussion about routine checks of the pneumatic system to verify all is OK.
It is a good discussion and raises a topic all owners should be aware of. Our pneumatic system has a wet tank (the first tank downstream from the dryer), a primary brake tank, a secondary brake tank, and an accessory or auxilliary air tank. Some coaches may have more than one accessory tank and those may or may not be part of the Prevost pneumatic system.
What we as owners need to know is the tanks are dry with the exception of the wet tank which if it has any moisture should be a tablespoon or so. We also need to know the protection and check valves associated with the tanks and system are functioning and how to check them without getting under the coach. You can to a degree, but it really isn’t necessary. First, all of our coaches have a requirement for a chassis lube at 6200 miles. That means someone should be under the coach at least that often. For most of us that is at least every six months. When the technician is under the coach that is when the system can be checked.
In addition to looking for moisture (which is bad) the tanks should be drained in sequence, starting with the wet tank to primary to secondary and to accessory. Drain completely before draining the next. Each tank in succession should have retained all its pressure. If not the check valves or protection valves may need service.
I also check going backwards. The accessory tank can be expected to draw air down from the brake tanks until about 75 PSI at which point the protection valve should come into play. 75 PSI is ample pressure for the brake system.
To do a check without getting under you can kind of do a check. If you apply and release the emergency brakes you should retain full air pressure on one brake gauge, but not the other. If you drain the accessory air you should retain about 75 PSI on the brake gauges. That’s not the perfect way to check but it does test most of the system.
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12 Replies
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Jon,
I’ve been to Prevost on more than one occassion and have always asked them to make sure the air tanks are dry. I can assure you they never do it in any kind of sequence and never check any pressure gauges while doing their tests. Of course, water is the primary reaso for tank draining.
Protection and check valves are seldom checked, in my opinion.
I’ve had pretty smart folks on this forum tell me in the past that aux air can’t affect the main brake system air. In some cases this is correct. In other cases, it can as controlled by the protection valve. Many not know where the check valves are located. I can assure you that the Prevost air drawing for my coach either doesn’t show the check valves or they can be seen. I did look at some very good drawings with explanations of comemrcial truck air systems. These make perfect sense. Sure, they don’t address aux air or air bag suspension.
There’s a tremendous amount of knowledge here on Norgren air control valves and methods of checking for leaks. Seldom are the protection and check valves discussed. This could simply be because they seldom fail. Regardless, they are a critical component in our air systems.
I remember someone hear saying it’s good to drain the water from the bleeder valve at the wrecker connection in the engine bay. After that I tried to do just that and zero air came out. I was assured it should. As it turns out this line is blocked when the dryer triggers full air pressure. At lower pressures, it will pass air. It’s important to know that water coming out of this drain is meaningless and fully expected. Keep in mind this is the same air that’s coming from the compressor before it has entered the dryer. It is important to drain this before applying air to your system from your shop air or a wrecker so you don’t needlessly push wet air into the dryer.
I’m certainly not an expert. Your explanation of cycling the emergency brake control doesn’t make sense. It should take air from either the primary or secondary air system. If it doesn’t work that way then it wouldn’t seem to be 100% reliable.
Here’s a great reference for the main air system: Bendix Manual
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The Schrader valve on the rear tail light post only sees pressure (as in from a tire gauge) when the engine driven compressor is supplying air. When it shuts off the pressure goes away. It is a filling point to pressurize the entire coach. I suppose you can get some moisture from the fitting, but that is not its purpose.
The accessory tank is filled from the system when the brake tanks reach 75. You can see that on your gauges. Watch as the brake systems are satisfied. Once they reach the protection valve pressure, the valve opens and all three tanks are taking air from the compressor at once. If you open the accessory tank drain or otherwise bleed air from the accessory tank such as a catastrophic air bag failure the protection valve will maintain 75 PSI in the brakes even though the accessory pressure is zero.
I think you are right about the emergency brake taking air from either although I want to verify that before I am positive one way or another.
Prevost techs should check the protection and check valves by draining tanks completely in sequence. If they don’t shame on them. That is the only way to verify the check and protection valves are functioning. I have heard of protection valve failures, typically as a result of moisture coming back to the accessory tank through the protection valve, eventually screwing it up so it wont open or barely opens. I have not heard of other valve failures, but given that so few people actually are checking the systems as specified by Bill Jensen and Robert Hitt it might not mean they are reliable, just that nobody knows. As long as you can drain one tank on the brake system and the other system does not lose air you know the safety valves are functioning.
My service interval is 5000 miles and to date on all my coaches I have never collected hardly any moisture nor ever had an issue with check or protection valves. The key is 2 year dryer cartridge changes and frequent water trap and tank draining.
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Okay, curiousity got the best of me.
I started with all air systems drained.
1 – Using the aux compressor the aux system was brought up to pressure. The primary and secondary systems stayed empty.
The shrader valve in the engine bay had no air pressure.
2 – I drained the aux system, all systems back to zero. I ran the engine and brought the main and secondary systems up to pressure. The aux system stayed empty until the main system got to about 65psi and then the aux also filled. After the compressor hit its governor pressure, there was no pressure at the engine bay schader valve. 3- All systems are up to expected pressure. The aux tank is drained and the primary and secondary tanks stay at their expected pressure.I use the parking/emergency brake to release pressure. With each activation and rest of the brake both primary and secondary tanks drop pressure evenly. At about 35psi the parking/emergency brake control popped.
So, I guess everything as I was expecting. Whether that is right or wrong, I’m not an air expert.
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You proved some important things. You proved your aux protection valve functions properly by not allowing pressure to enter it until the brakes are satisfied. You proved a loss of aux air leaves your brake systems pressurized. You proved your emergency brake will engage if you lose brake systems pressure.
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That’s exactly how it’s suppose to work and your’s is all good Gil.
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Are we saying that the two pressure reading at the dash are primary and secondary readings? I know they are both brake system readings.
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Yes.
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I have no idea why they are referred to as Primary and Secondary as the share air supply requirements equally. They are sometimes referred to Front and Back, which may be more accurate and certainly not as misleading as Primary and Secondary.
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The primary is the rear brakes. I would far rather lose my front or secondary brakes if given the choice so maybe that is how they got named.
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I thought that the majority of braking takes place in the front brake system. At least on cars.
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On cars the majority of the work is because the weight is typically biased towards the front, and then when a car is braked it shifts even more to the front, which can be seen when under heavy braking the nose dips and the tail lifts. In a car you also have 2 wheels per axle.
Our bus has the concentration of weight in the rear plus 3 times the number of wheels at the back compared to the front.
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There’s practically no fore and aft weight transfer on a long wheelbase bus such as ours. Cars require anti-squat and anti-dive suspension geometries – no need for this on a bus. Put in engineering terms, the pitch moment arm is small in relation to the length of the wheelbase resulting in minimal squat or dive. Braking power is proportional to the static weight distribution; that is, the drive and tag axles do close to 70% of it. Lucky for us 75% of the rubber contact patch is at the back!
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