Air Brake Test

[wagwar]

1981 MCI MC 9. Air suspension stays up for weeks, other air systems will leak down in a couple of days.

I run air brake tests fairly regularly, but I recently fixed some air leaks so I wanted to see if that improved the results. It has fixed the leak down issues and improved the air build-up time, but I may still have some problems. I'd appreciate your advice as to: priority, cause, repair.

Air leakage w/ engine off and with foot brake applied for 60 seconds is 0 (zero)
Air warning light and buzzer comes on around 50 psi vs. the 60 psi I was told was correct.
Emergency brake applies automatically at around 15 psi vs the 20 - 45 psi I was told is correct.
Air pressure buildup from 85 psi to 100 psi takes 1:40 vs the :45 I was told it should be.
Governor cuts of at 125 psi
Emerg. and Service brakes prevent vehicle moving when applied.

So, I still have some issues, I'm just not sure how serious they are and what I should do about them.

[eagle19952]

Air pressure buildup from 85 psi to 100 psi takes 1:40 vs the :45 I was told it should be.....

this would concern me the most...I believe that I would at the least remove and flip the valve plates in my compressor and re assess the time to top up the air tanks...I think that 85 psi to 120 should take 45 seconds or less...
and I would inspect the unloader  portion of my compressor.

[luvrbus]

85 to to 100 psi in 45 seconds @ 1800 rpm not idle,low pressure warning depends on the manufacture Prevost come on at 66 psi,parking brakes depend on the type springs set at 40 or below and that can depend on the PP1 valve they range from 15 to 40 psi setting who knows which setting you have it may be a 15 psi pop and not a 40 psi 
The parking brake should hold on a 20% grade or not move with the engine at 1100 rpm in low gear 

You do have a problem with the drop of pressure with the brakes applied that is way over the limit or are you saying you loose no pressure after full application ?

A 500 Bendix compressor will take longer to build pressure than a  700 or 750 which most tests are based on    

[Oonrahnjay]

  Air pressure buildup from 85 psi to 100 psi takes 1:40 vs the :45 I was told it should be.....

this would concern me the most...I believe that I would at the least remove and flip the valve plates in my compressor and re assess the time to top up the air tanks...I think that 85 psi to 120 should take 45 seconds or less... and I would inspect the unloader  portion of my compressor. 

     My bus does not have a "high idle" - I find that the time to build up air, especially the high-end build up that you're talking about here, varies a lot by engine RPM.  I don't have a tachometer (yet) but I find that if I hold the accelerator just a bit open and let the engine rev just a little, it makes a lot of difference.  You might want to get the RPM specs for your bus during this test and reassess. 

     Your system seems pretty much right except for this and if this is the only thing out of line, it might be your test specs.  I'm going to guess that if you re-test it, you're going to find that it will meet the requirements OK.  (But of course, for safety's sake, you will have to retest carefully.)

     (And, yes, if the correct test shows that it builds OK at low pressures* but then slows down too much during the high-end test, the advice to look at the compressor, compressor valves, and governor/unloader system sounds right to me.   *You don't mention how if builds up but you didn't mention slow buildup at low pressures, so it would seem that you haven't noticed a problem there.)

[bevans6]

Here is the test procedure I was taught, and use.  http://www.mto.gov.on.ca/english/handbook/airbrake/section10-3-0.shtml  The test name is "compressor recovery".  You go down to 80 psi and time from 85 - 100 so you don't have any lag from the governor turning on to confuse the test.

Basically under 2 minutes, 600 - 900 rpm.  With that said, a bus with a normally working compressor will recover in 30 - 40 seconds at 900 to 1000 rpm, or a typical high idle.  Mine, with a fresh rebuild Bendix Tu-flo 750 recovers in about 20 seconds.  It is important to have all accessory air users off or not drawing air, so the suspension should be up and stable, etc.

The rest sounds pretty OK.  The gauge is not so accurate that I would worry about the exact pressures that things pop or come on at, as long as they are close.

Brian

[Jon]

On any test of pressure recovery time make sure the aux air system is fully pressurized because the DOT standards are for a vehicle with a very limited aux system, unlike our coaches which can have a large capacity aux system.

Ditto on the increase in revs.

[TomC]

I wouldn't worry about it-you're not a commercial vehicle. You don't have leaks, and the compressor is getting up to 125psi. Good Luck, TomC

[eagle19952]

The main reason to be concerned is that IF you ARE still having a problem AFTER you recheck your times USING road speed RPM's AND your recovery times remain the same...IF you find your self in a situation where you absolutely MUST  make multiple brake applications in a very short period of time AND you are MORE than 1 minute and 40 seconds away from running over a young mother pushing a baby carriage....then I wouldn't worry about it either....

[wagwar]

Air pressure buildup from 85 psi to 100 psi takes 1:40 vs the :45 I was told it should be.....

I should have said this is at engine idle (750 rpm). At speed, times are much shorter.  So, 1:40 'at idle' is still within spec? (less than 2 min.)

On the leak down, I meant that there is 0 (zero) pressure loss.

While driving, I never have any problem maintaining 115 - 125 psi. 

[bevans6]

The DOT rules for air brake system worthiness do not differentiate between commercial and non commercial vehicles.  It's like your kid is bringing home a math test and gets 51%.  Pass, yes.  Acceptable - maybe.  Want to do better - absolutely!  Gonna try harder to fix what's wrong - no question.

In the OP's case, given that it's a 1981 MCI MC-9 and while you can work on the compressor while it's installed it's a PITA, I would get me a nice Bendix re-manufactured unit and change it out, for the $500 it costs.  I pulled a perfectly good yet unknown age compressor off my engine while it was out and put a new one on, just because I have an MC-5C and changing the compressor with the engine in the bus is a job for a man with more patience and less girth than I have...

Just read your post.  Your engine probably is idling lower than 750, mine is set to 550 for example.  Raise the speed to 1000 - 1200 and repeat the test.  If you have the original compressor it may be a 550, so will tend to be a bit slow.

Brian

[luvrbus]

DOT test here is 1800 rpm testing for recovery I thought that was the standard nation wide in the USA ?

[bevans6]

When I've searched in the past I've seen different speeds and different times (time as low as 45 seconds for one State  up to two minutes) based on the State or Province.  The link I posted was Ontario, since that is where I took the training that's what I am most familiar with.  I don't know if there is a Federal standard, or just State standards.  Up here it is regulated by Province, not Federal. 

Brian

[akroyaleagle]

All of the questions we ask have Official answers.

Questioning legality of whether or not some of the laws pertain to us is best left to lawyers.

Most guidance is safety related by nature and should be of great interest to us.

Here is the DOT law. The part this thread is asking about starts at page 120. You may look in the index, then just click the subject or slide the bar down on the right of your screen instead of clicking the down arrow. Take the time to scan through the entire document when you have the time. It contains a great deal of info.

http://www.dot.gov.nt.ca/_live/documents/content/Air%20Brake%20Nov%202007.pdf

[bevans6]

That's the law in Northwest Territories, Canada...    Good for ice road truckers, that's for sure!

Edit - that book has a completely different compressor recovery test.  They want to see the compressor recover from 50 psi to 90 psi withing 3 minutes.  They don't ask for the 85 psi to 100 psi test at all.

Brian

[akroyaleagle]

Bevans6:

Here's the South Dakota Test: The brake pretrip test starts on pg 137.

http://dps.sd.gov/licensing/driver_licensing/documents/cdldrivermanual3-16-10final_000.pdf

This the same test as Alaska.

I chose the to post the first test because I thought it was presented better. That reference is, in my opinion, more easily understood by non Commercial Drivers. It also includes a lot of safety info not normally contained in one document.

Again, I do not wish to engage in trivial legalities but only to present verifiable information for the original poster and any other interested folks. Legalities are matters for lawyers and Safety Directors of Commercial truck and bus companies. My interest is Safety, Safety, Safety.......

[eagle19952]

Again I don't think DOT cares what your cycle speed/time is at idle...
Operating RPM...they probably do.
As you say @ road speed RPM you have no problems, then your good.
The difference at idle lies in what CFM/model compressor you have, obviously bigger is better...gear driven better than belt etc.