How long should your air last?

[Uglydog56]

I finally finished the repower on my Crown tonight and got it off the blocks.  My batteries get here tomorrow afternoon and I'm going to see how bad of a mechanic I am shortly thereafter.

Once I had it on the ground, the pressurized the air system with my little electric compressor to check for leaks, since I had changed the compressor configuration.  It pressed right up, but was down to about 30psi 2 hours later.  I looked around and found my fast idle was on and it had a leak, and that I hadn't tightened one of the fittings on the transmission hi-lo switch.  So I fixed those real quick and pressed it back up.  I don't have air ride.  I replaced a lot of lines, but didn't get them all.  I know it's pretty difficult to get these old buses air tight.  How long does the air pressure have to last to be considered acceptable?  Thanks.

[bevans6]

The DOT spec is 3 psi per minute with the brakes fully applied.  You lost say 90 psi in 2 hours, 3 psi per minute is 90 psi in 30 minutes.  It's legal, but it's still a lot more than I am used to seeing.  My bus tends to bleed down that far in 12 hours or so, actually sticking at 60 psi due to the protection valve.  On the road I tend to cycle the compressor about every 30 minutes in normal highway driving.

Brian

[belfert]

I don't know what normal is, but my bus will generally keep the air above 80 PSI for weeks in the summer months.  It will bleed off in a day or two in the winter.  I only have airbags on the tag axle.

[Lin]

My dash gauge is off the accessory tank, which will bleed down to zero within and hour.  I believe that is due to leaks in the wiper valves.  I have a separate gauge in the engine compartment that drops to 85psi as the accessory tank empties, but then just stays there for a long time. 

[Uglydog56]

My gage has two needles, a red one and a white one.  The way this was explained to me, was that the white one was the "normal" air and the red one is the emergency brakes?  I don't know, the previous owner did it.  It originally had two pressure gages up front, he changed one to this dual gage, and hooked the other one up to the brakes so you could see how much brake pressure you were using.  I'm going to press it up and pay closer attention today.  Is it 3psi per minute with the park brake set, or brake pedal held down?  I will have to get my wife to hold the brakes while I crawl all around underneath with my hearing aids turned up.  I wish I knew more about air brakes.  I already talked to the local truck/rv place here; they are going to do a safety inspection and brake tutorial with me as soon as I can get it there.

[Oonrahnjay]

My gage has two needles, a red one and a white one.  The way this was explained to me, was that the white one was the "normal" air and the red one is the emergency brakes?  I don't know, the previous owner did it.  It originally had two pressure gages up front, he changed one to this dual gage, and hooked the other one up to the brakes so you could see how much brake pressure you were using.  I'm going to press it up and pay closer attention today. 

     It really isn't necessary but I have two dual guages, a readout for my "wet tank", a readout for my main tank (rear brakes), a readout for my secondary tank (front brake circuit), and one for my accessory tank.  The main tank (and its air supply to the service and parking/emergency canisters on the rear axle and to the brake valve on the dashboard) is really the important reading but I feel that watching the pressure rise when airing up and any difference in drop is a valuable set of information when I'm driving and also it will be helpful in troubleshooting.

Is it 3psi per minute with the park brake set, or brake pedal held down?  I will have to get my wife to hold the brakes while I crawl all around underneath with my hearing aids turned up. 

      There are different specifications for "brakes applied" and "brakes not applied" (I'm not sure if these are DOT inspection standards or industry engineering norms) so I'd suggest that you investigate this one and get complete data and also context so that you can be sure that you understand what you're considering.  If I have any question about leaks or any system malfunction, I do both.

I wish I knew more about air brakes.  I already talked to the local truck/rv place here; they are going to do a safety inspection and brake tutorial with me as soon as I can get it there.

       Also, have a look at possible Bendix air brake school locations near you.  It requires a few days and $300, but it's very complete and well worth the investment.  It covers everything you'd need to know when operating an air brake bus.

[bevans6]

Glad to hear you are going to get a tutorial and inspection - the thing about air brakes is that they are "active" brakes - a lot has to be working right for them to do even the first thing about stopping, compared to hydraulic brakes that can work pretty good with absolutely nothing working but the pedal.  So you need to know exactly what should be happening so you can tell if it stops happening before you need the brakes "right now"...

The test is part of a DOT daily inspection for all air brake equipped vehicles, it's just have the air at 100 - 120 psi, engine off, suspension fully up, all accessories off, foot fully on the brakes, parking brake off, and see what happens.  If anything bleeds down, you should notice, the US and Canada federal test rule is 3 psi per minute for a bus (different for a straight truck and a truck/trailer combo).  It is a test of the full air system including the service brakes, but not (on DD3 systems like mine) a test of the emergency brake system.  Being a DOT test, you can be pulled and tested at any time on any road regardless of if you are private or commercial.  Not that that happens all that often, I suspect.  AS part of your tutorial/inspection, you should figure out exactly what part of the air system your gauges are connected to.  Dual gauges are usually for more modern spring brake systems that have separate front and rear tanks, one gauge for each tank.  Some buses have the gauge connected to the accessory system - it usually bleeds out first and takes the service tank down with it to the point where a protection valve operates.  My bus has the gauge on the service tank, AKA the "dry tank"

Edit:  Agree with what Oonrahnjay wrote.  Where I live, you need to have a license to operate a vehicle with air brakes so there are many trade schools that teach the course.  That's the course that I took, and I have to re-write the license test every two years and have a medical for the license to drive the bus.

Brian

[luvrbus]

Most gauges on your vintage year of coach 1 needle is for air pressure and the other was for brake applied air pressure the original 05 Eagle dash had the 2 needle gauge for the air system 

[akroyaleagle]

Try this. It is the DOT test!

AIR BRAKE TEST SINGLE VEHICLE

With ENGINE OFF, AIR PSI at MAXIMUM, wheels chocked, spring brake released,
and key on.

1)  Air Leakage Rate

   a. Watch the air supply gauge for 60 seconds, the air loss should be no
       more than 2 psi
   b. Apply the foot valve fully, Watch the air supply gauge for 60 seconds,
       the air loss should be no more than 3 psi.

2) Air Warning Light
 
  *Apply and release the foot valve until the air warning light comes on, this
    should happen before 60 psi.

3) Spring Brake

   *Apply and release the foot valve until the spring brake pops out, this should
     happen between 20 and 45 psi.

4) Air Build Up Rate

   * Start the engine to build air pressure, when air pressure reaches 85 psi, time
      build up rate to 100 psi at idle. It should take no more than 45 seconds.

5) Governor Cut Off

    * The Governor should cut off at about 110 to 125 psi.
       You will hear air release from the system.

6) Spring Brake

    a. Remove the chock blocks.
    b. Place the vehicle in drive.
    c. Attempt to pull the vehicle forward, the vehicle should not move.
    d. Place the vehicle back in neutral.

7) Service Brake

    a. Release the spring brake.
    b. Place your foot on the foot valve.
    c. Place the vehicle into drive.
    d. Put the vehicle in motion, when the vehicle reaches the speed of 5 mph
       apply the service brake to stop the vehicle. The vehicle should stop!

[Uglydog56]

Thanks for the info, guys, I really appreciate it!

[Oonrahnjay]

Try this. It is the DOT test!

     Yes, that's complete - and as I mentioned before, each of us who owns a bus should get enough background knowledge to know what to expect and how to recognize some aspect of the system that's not right as we're going through the tests.

     I'd also note that it's important to do these checks often.  I had to almost completely replace the brake system on my bus (lines, tanks, protection valves, brake cans, etc.) - when I finished I aired up to 120 with shop air; it dropped to about 116 or 117 psi after an hour.  I tried again a few days later, forgot to check after an hour and it was still holding more than 115 after an hour and a half.  That felt pretty good to me; then I got involved in a few projects for a few weeks that made it impractical to start my bus but when I did, I heard some small leaks; I aired up to 120 again and this time I dropped about 20 pounds in an hour.  So, once you get a system pretty air tight, things can change!  Based on what I found chasing leaks, I'm pretty sure that any changes were just "settling in" but it took finding them and tightening them.

[luvrbus]

Most states have different tests for a vehicle under 29,000 lbs and older systems the Federal Dot test is not per say here in Az or some other states  

The newer stuff have such large tanks and high capacity cfm air compressors it hard for one to fail just keep your system in good shape if you hear a leak fix it  

I doubt the DOT will ever check a motor home or there would be a lot of RV setting with the pretty red tag in the windshield air pressure is just one small part in a complex system to me   

[RJ]

I'd also note that it's important to do these checks often.

Yup, it's called the "Daily Pre-Trip Inspection," which is required of all CDL drivers every day if they're driving the same vehicle, or every time they get into a different vehicle.

I spent quite a bit of time talking about this in a seminar at the Arcadia rally several years ago, trying to emphasize to the folk in attendance just how important this is.  Granted, 99% of those in attendance do not have a CDL, but the principal of the pre-trip remains the same when you're driving an air-brake equipped vehicle.

Did it sink in?

Nope.

On the last day, when most of the folk were leaving, I walked around, listening.  Out of the 100 or so coaches that were there, I only heard ONE doing an air brake check.  Most folk at least did a walk around looking at lights, retrieving their electric and sewer, checking the oil, and other pre-trip related duties, but again, I only heard ONE coach operator doing an air brake check.

Sad. . .

 

[akroyaleagle]

Please read this post in the spirit with which it is written. I do not mean to offend anyone, only try to educate.

I too, RJ, have spent a considerable amount of time beating this horse. I am appalled by the folks that buy these buses
who have never had any airbrake training. Most may have never driven anything bigger than a pickup!

I have been Director of training and safety/driver trainer for a bus company
and Supervisor/driver trainer of a large fleet of commercial vehicles for a GSA motor pool.

I never checked a new driver that had a CDL that performed the check correctly on the test drive.

It was one of my areas  that I placed emphasis on.

While I don't do it everyday in my own coach, I perform it often. I am the only driver of my coach.

Folks, there is no substitute for doing this correctly. It is the only way to ensure the performance of the system.

I have never looked at an 01 conversion (that did not have the brakes upgraded) that could pass this test. nearly everyone of them
were dangerous (in my opinion). Most of them could not have avoided an accident at low speed by getting the coach stopped.

Please, if you do not understand this test, get someone to help you. It's pretty easy. Just do it as my other post indicates.

[Lin]

Although I am not minimizing the importance of checking the brakes before starting out, I will say that when I air up, release the parking brake, and begin to move, I am trying to be aware of anything that may be different or amiss.  On the one mile dirt road from my house to the highway, I believe I have a clear picture if anything is wrong.  I think that one can say there is a difference in familiarity between someone who is assigned a vehicle and a bus nut that is working on the coach often and just driving that one.