draining air tanks questions, please help!

[bevans6]

I went for my first drive in the bus today - 1980 MCI MC-5C, and like a good little newbie I am doing the full after-drive session on the air tanks and system after my long, exhausting 10 mile drive  .  I drained all the tanks first - first question - is the wet tank the drivers side or curb side of the two tanks in the front axle bay?  

I drained the drivers side first, since I thought it was the wet tank, but got more moisture out of the curb side tank, which I drained second.  Then I drained the dry tank, which is behind the wall of the drivers side rear cargo bay.  Fine, no moisture at all.  Then I went to drain the accessory tank, but it was already fully drained.  I didn't expect this, I thought there was a check valve to keep it up.  Of course, it would probably make ore sense for the check valve to keep air in the dry tank in the event of a failure somewhere in the accessory system.  But there is a pressure protection valve whose function I don't fully understand between the dry tank and the accessory tank as well.  Perhaps all it does is limit the pressure in the accessory system.

Anyway, couple more questions.  At no time did I sense or measure that the suspension was settling.  Even with no air in any tank and the pressure gauge on the dash reading zero.  So are the ride height valves keeping the suspension up, do they act like a check valve to the bags?  Next question - where does the dash pressure gauge measure the pressure from?  If the accessory tank is limited to 65 psi the way I think the pressure protection valve works, it won't be reading the accessory tank pressure.  i would expect that it would be reading the dry tank pressure, is this true?

Measured the time it took the compressor to pump up everything from all tanks at zero, 5 minutes and 30 seconds.  Measured the time to recover from 85 psi to 100 psi at high idle, took 25 seconds which I thought was very good.  The truck we took our air brake test on took 40 seconds.  Compressor cut in is at 95 psi, cut out is at 120 psi for a 25 psi range, dead on what I was taught.  Pressure loss on application is near enough zero over  3 minutes, engine off.  Now I have to figure a way to measure the slack travel and I will be all set.

Please let me know what you think, and what I'm missing.  One thing I don't know is if the parking brake should auto-apply at  low air pressure and how to test that.  It works fine normally - applies fine and releases fine.  I apply it by pulling the knob and pressing the pedal, I release it by pushing the knob then pressing the pedal again.

Lots of questions, I appreciate any and all feedback!

Brian

[JackConrad]

I think the check valve is to prevent the accessory tank from fillling until the other tanks have 60-70 PSI. However it allows air to return to the other tanks if the pressure in them is lower  Jack

[Len Silva]

I'm not familiar with MCI, onlt GMC, but the wet tank will be the first tank after the compressor, probably in the rear.  Secondly, the aux tank is not limited to 65 psi, rather it does not start filling until the main tanks have reached 65 psi. And yes, if everything is perfect, the suspension will stay up even with all the tanks drained. (Perfection is such a difficult state to reach).

[buswarrior]

I expect the tank with more moisture is your wet tank, should work the same next time.

The aux has sacrificed itself back to the other tanks, as noted.

The air gauge on MCI of that vintage reads off the aux, thinking is that the rest has to be full to get the aux full.

Nice that it passes leak down.

Pump your brakes down, see if/when the knob pops, and as you reach each 10 pounds of drop below 80, see if it will move, drop another 10 and see if it will move...spring brakes are on before the knob pops, the DD3's behave a little differently.

And, maybe best to not go for a drive again until you measure your push rod stroke?

happy coaching!
buswarrior

[gumpy]

MCI wet tanks are in the front, as is the dryer. Not sure which side, though I would go with the one with the most moisture in it.