Pressure runs too low on MCI-5C

[chuckdrum]

The air pressure on my '79 MCI-5C generally runs between about 85/90 psi on the low end to about 115 psi at max.  On the way home from a recent trip I noticed my pressure dropping below that while under way.  I pulled over when it was approaching 55 psi and the emergency brake actually engaged on its own by the time I reached a full stop.  (note: earlier on the same trip I noticed the air release sound lasted longer than usual on a couple of the cycles and it was taking longer for the pressure to start back up from 85)

The pressure gradually started to build again on its own and I continued driving (carefully and closely monitoring the pressure gauge).  The pressure built up again to around 115 then started to come way down again.  I have a separate compressor plumbed in so I turned that on to bring pressure up again.  The aux compressor is on a timer so the Mrs. had to keep turning it back on for the remaining hour or two of our trip.

I assume there's a governor that regulates that pressure?  Is that likely the culprit and needs to be replaced?  Serviced?  Where is it located, exactly?  Thanks for any insight and assistance.

Chuck

[chuckdrum]

I just now stumbled across Brian's (bevans6) thread from back in '09 regarding "How to test everything on brake systems". I haven't deliberately done many of his suggested tests, but it's possible I have multiple issues going on as I doubt I could pass all of those challenges. Thanks for that info, Brian.

[richard5933]

...I assume there's a governor that regulates that pressure?  Is that likely the culprit and needs to be replaced?  Serviced?  Where is it located, exactly?  Thanks for any insight and assistance.

Chuck

This part of your post has me concerned...not about your bus, but about you not knowing about the pressure governor on your compressor. I understand that not everyone is a mechanic, but to me it seems vital that everyone driving a vehicle with air brakes has a working knowledge of the various components in the system. Not trying to be harsh here or to pick on you, but anyone driving a vintage bus should really read up on how air brakes operate.

I'm not saying that we all have to have enough knowledge to do repairs, but at least enough to know the basics. My first suggestion is to get a copy of the manual for your bus and read through the air brake section. If you don't have a manual for your particular bus, search for one of the Bendix publications which lays out the general operation of air brakes. You may not know where the governor is when you're done reading, but at least you'll have a general sense of what it does.

Back to the current issue...

I'd guess that you have a leak or possibly a failing compressor. First check should be a standard leak test. With bus on level ground and the tires chocked, bring the pressure up to cutoff level, shut off engine, and then release the parking brake.

Have another person outside the bus listening for leaks at all four corners.

Turn the key on (or switch) but don't restart. Press and hold the service brake pedal and hold for one minute. After you press the pedal the needle on the pressure gauge should stabilize, and then you should have no more than 3 psi of loss over the one minute you're holding the pedal.

After the minute, pump the pedal to lower the air to test the dash light and the buzzer. They should come on before it gets down to 60 psi to warn you of low air. Then keep pumping to test if the emergency brakes apply before you get to 25-30 psi.

Turn the engine back on, and once the pressure gets to 50 psi it should build back to 90 psi within 3 minutes at idle.

After you do the basic test, you hopefully will have some idea if you have a leak or a weak compressor.

Sounds to me from what you described as a leak situation that your compressor was unable to keep up with. Dangerous to drive, for sure, until you get it repaired. If work hasn't been done on your bus in a while you might need to start changing out air lines - they get hard and brittle as they age. Lots of places to get those made for you.

Regarding the cutoff pressures - hard to tell if your air pressure gauge is accurate, but generally speaking the compressor should kick on at or above 90 psi and should cut off at about 120 -125 psi. If it's not hitting those numbers, then you can get a new governor. They are not expensive and are not that difficult to change out. Luke at US Coach should have one on the shelf if you need one.

[Bill Gerrie]

 (note: earlier on the same trip I noticed the air release sound lasted longer than usual on a couple of the cycles and it was taking longer for the pressure to start back up from 85)
If you have an air dryer it sounds like you have a leaking valve on it once it purges it doesn't completely close. Read Richards post and DO IT. If no air dryer it will almost for sure be a bad D2 governor.

[DoubleEagle]

Another item that can cause slow buildup is the screen that is used in the air intake hose being clogged. The governor is a common problem, but so is a tired old compressor being ready to be replaced. A healthy compressor should be able to go from 85 psi to 100 psi in five seconds @ 2100 rpm. Dozens of little air leaks can equal one big one that you can hear, but will have you crawling all over to track them down.

[HoBoJo Busman]

IE;   AIR BRAKES-   I understand that not everyone is a mechanic, but to me it seems vital that everyone driving a vehicle with air brakes has a working knowledge of the various components in the system.

Reading your posts here on this forum Richard, has me appreciating you more each day.

Thank you for your thoughtful contributions to this fine forum and know that we appreciate your involvement and constant interesting comments and questions which help us all to learn about our varied buses...truly.

Your thirst for knowledge about the coach you own, is contagious.  I'm hooked !

[chuckdrum]

Richard-
I appreciate your candor and I am deserving of your constructive reprimand.  Yes, I should have that basic brake knowledge.  And I've gained quite a bit over the past 24 hours!

I have some other non-bus stuff I have to complete today and will not have time to test the brakes for a day or two, but I already know that
-the cut off is around 115, not 120-125 (I don't recall it ever being 120-125, but I might be mistaken there)
-the cut in is below 90 (on this last trip, WAY below!)
-the pressure will not go from 50 to 90 in less than 3 minutes
-I have no low pressure warning light

I'll be making an appointment at a local shop to (likely) have the compressor replaced and have them look over the condition of the lines.  The shop I've dealt with most recently is less than 5 miles away and I believe my aux compressor will enable me to get there safely, as it successfully helped me limp home this past weekend.  The pressure didn't drop THAT fast, even in my most recent experience.

[chuckdrum]

Update-

I ran the test Richard described above multiple times.

-cutout was at about 111psi (my gauge has 10psi increments, and it was just beyond 110)
-with engine off, parking brake off, service brake held for 1 minute, there was minimal pressure loss; minor hiss from the front somewhere and the aux compressor leaked until its gauge was around 110~115 (read differently than dash gauge) and I assume this is its own safety valve
-recovery from 50psi to 90psi was 3 minutes or less on all tests (averaged around 2:45-2:50)
-parking brake did NOT engage at all on multiple runs, even dropping to essentially zero pressure.  This runs counter to my experience on the road this past weekend when it kicked in around 50psi.  It finally did that today after multiple attempts.  When it IS engaged, it won't release until pressure builds above 50-55psi, as per usual in my experience.

I also tried leaving the engine running, parking brake off, and feathering pressure down to see when the compressor would kick back in.  It did not, until I got to 55~60psi... at that point I discovered hissing from what turned out to be the air dryer.  It went on for about 3~4 minutes then stopped and the pressure began to build again.  I don't specifically recall the sound or its timing from last weekend's experience, but the delayed return to pressure build-up was the same, and at about the same pressure.  I also noted that if I did not allow the pressure to build all the way to the cut off point, and feathered the pressure back down to, say 65~70, it would build up again.

This makes me think the problem is with the governor rather than the compressor or air leaks, though I'm not clear on how that relates to the air dryer.  The parking brake thing is a bit of a mystery (and unsettling!), as I haven't had that happen before.  It still functions fine when manually engaged, but didn't do so automatically today until several trials into it.

Thoughts?

[richard5933]

One question first...

You did this test with the auxiliary compressor and tank out of the system, correct?

If so, sounds like you might have a couple of issues. Governor would be my first thing to replace - it's relatively inexpensive and if you have a cutout of 110 psi it sounds low. Someone else more familiar with the MC5 can confirm, but it seems that this should be at least 115, or preferably 120 before cutout.

If you have a hiss from up front when you are applying the service brakes, something is leaking. Have a helper hold the pedal while you figure out where it is. Possibly a loose fitting, possibly a failing service brake valve. Either way, needs to be fixed as it won't get better on its own. Assuming that this is separate from the air dryer hiss.

If your e-brakes are not consistently self-applying when pressure is at 20-30psi, the control valve might be failing. Another easy-to-fix problem that can save your life in the future. It might also be where the hissing you hear is coming from.

All that said, your governor might not be the problem or the only problem, if you hear a hiss from the air dryer. Make sure the signal air line between the air dryer and the governor is not leaking and that it's not clogged. Also check the valve on the air dryer - these sometimes stick partially open. Could be a piece of something stuck in there, or it could be a failing valve.

On all the valves, check with Luke at US Coach. He probably has replacements, and he might have rebuild kits.

[buswarrior]

You are dealing with an older DD3 brake system, parking brake test results under low air pressure aren't what you might expect them to be, with the parking air isolated from the service air.

So long as cut-in and cut-out of the compressor is consistant, it is just an adjustment of the air compressor governor, to get a higher cut-out pressure. The older stuff didn't run hard up to 120, like the somewhat newer stuff does.

When feathering the pressure down, how long were you leaving it , to detect whether the compressor was trying to pump up again? The air compressor governor should faithfully and consistantly cut-in somewhere 20 to 25 lbs below the cut-out pressure, like a religion, like a clock, the same every time...

Find the leaks, and make that air compressor governor prove it is working right, or replace it.

Happy coaching!
Buswarrior

[6805eagleguy]

As somewhat mentioned already, I believe governor controls air dryer

[richard5933]

You are dealing with an older DD3 brake system, parking brake test results under low air pressure aren't what you might expect them to be, with the parking air isolated from the service air....
Happy coaching!
Buswarrior

Not sure what you're saying here. Are you saying that the e-brake won't self apply when the pressure drops to the 20-30psi range? My DD3 brakes do that every time I pump down the brakes to do the pre-trip.

[hogi6123]

Not sure what you're saying here. Are you saying that the e-brake won't self apply when the pressure drops to the 20-30psi range? My DD3 brakes do that every time I pump down the brakes to do the pre-trip.

My DD3 parking brakes do the same as chuckdrum's (in a MC-9).  What I understand is happening is:

- There is a separate tank reserved for applying the parking brakes.
- The service brakes pull from the main tank until it is low.
- When the main tank is low, the service brakes then pull from the parking brake tank.
- The parking brake will apply automatically when the parking brake tank is low, around 25-30 psi I think.

There is no pressure gauge for this tank on the dashboard, so it is disturbing for those new to this setup.

To test the automatic application of these brakes, you have to keep pumping the brakes well below 20 psi on the gauge, eventually the parking brake tank will empty also and the parking brake will apply.  Probably best way to test this is park on a gentle slope and if the bus doesn't roll after the parking brake applies, it is working correctly.

It's possible I'm wrong but I don't think so.

[chuckdrum]

"Probably best way to test this is park on a gentle slope and if the bus doesn't roll after the parking brake applies, it is working correctly."

Well, yeah, that's one way to test...depending on the slope and the brake, it might be the last test you'll do with that bus!

I pumped it down to nothing on the dash gauge, no further air sounds or resistance on the pedal, and the parking brake knob didn't pop out.  I'm referring to the yellow push/pull parking brake knob.  It's pretty obvious if it's engaged or not.  That's what you're referring to, correct?

[richard5933]

If your gauge is getting below 20-30psi and the e-brakes have not self applied there is something wrong. On my GM 4108 the manual says the DD3 brakes will self-apply when pressure drop below 40psi, and I assume yours will be similar.

The parking brake button should pop out every time you pump the brakes down when they get below the set point. It's what will keep you alive when you have a sudden loss of air pressure. That's why it's part of the pre-trip inspection.