Parking Brake valve MC8

[rcbeam]

I am still fooling around with my leaking QR1 parking brake quick release valve and other air brake leaks.  Last weekend replace the push-pull valve.  When I go through the pre-trip inspection list that I am using, it mentions to pump down the service brakes and use up the air to test if the push-pull valve POPS.  Neither the old one or the new one will POP on it's own by pumping down the service brakes with the engine off. 

I have read the section in the manual and looked at the diagrams until I am crazy trying to figure out what to do next.  The slight leaking out the QR1 makes me think either bad inversion valve or a leaking rear brake can.  Now that the push-pull valve does not POP on it's on, makes me think maybe the inversion valve is bad... isn't the purpose of the inversion valve to POP the push-push valve when air is too low?

I hope by this weekend, to be able to jack it up, remove a set of duals on one side and get in there to look around.

Anyone with ideas here?

[thomasinnv]

just to point out the obvious here, but when you do the pump down test you do have the parking brake released right?

[buswarrior]

You don't have spring brakes, so don't expect your DD3 to respond appropriately.

"Knob popping" is another one of those silly tests instituted in the past by authorities who didn't know what they were doing. By the time the pressure is low enough for the knob to pop, the springs are already applied. The test is useless, well, no, it confirms the spring in the push-pull valve is still doing something, but to what end?

Anyway, back to your brakes, which are DD3's.

Plan to replace the inversion valve and QR1 and call it preventive maintenance.

Now, if you really want to get the push-pull control valve to pop in a DD3, you need to get the air pressure at the valve down below 40 lbs. Back to your diagrams, will fanning the brakes change the air pressure at the push-pull valve?

See what a spring brake world does to a DD3 mind?

happy coaching!
buswarrior

[bevans6]

As I understand it, the push-pull valve will pop up  on it's own when the pressure in the emergency tank is below the level required to overcome the spring pressure, around 40 PSI.  There are two ways to get there - one is to drain the wet tank, then the dry tank, then the emergency tank.   When the pressure in the emergency tank falls below the 40 PSI, the push-pull valve will operate and the parking brakes will be applied with the remaining air pressure in the emergency tank which whould be enough to bring the bus to an emergency stop.  The second is to fan the brakes down until the pressure in the dry tank is 40 PSI below the pressure in the emergency brake tank, which should cause the shuttle valve to operate, then keep fanning the brakes.  This should cause the E-9 valve to apply the emergency brakes, which is the E-9 sending pedal modulated air pressure via the shuttle valve to the emergency brake ports on the DD3's.  Eventually you will exhaust both the pressure in the wet/dry tank and the emergency tank to the point where the emergency tank falls below 40 PSI and the push-pull valve should operate as above.

Am I right?  Is there a prize?

Edit - based on my 1980 MC-5C schematic...

Brian

[buswarrior]

Depends, does the coach in question have a shuttle valve?

2 more variables that are not well understood:

What will the spring in the inversion valve do?
What will the spring on the locking rollers in the brake chamber do?

Unintended operation when conditions are less than "operational"?

As for the plumbing, there are wild variations back and forth over the mid 70's.

8 different air schematics for the MC8 in my book.

Government regulations before technology was ready, (anti skid control) revert to old, back partially to new, as well as localized state safety regulations from Mass and NJ...

And being a TMC, a little harder to choose which schematic to stare at, as the info is not as widely distributed.

Gotta try to cross some MCI unit numbers by month of build to get close.

rcbeam?

happy coaching!
buswarrior

[rcbeam]

All I can say is my bus is MC8, manufactured APR 76.  I have fanned the service brake until the guage on the dash basically read 0 and the push/pull valve did not POP.  Now if i nudged it a little then it did, but I always thought it was supposed to do it on it's own.  When researching wiring issues, I have found that the schematics don't exactly fit what is on my bus.  Mosty I find wires with the metal tags and can't find the tag number in the book.

I guess at this point, luvrbus has the best idea.  I have already replaced the QR1 and the push/pull valve, so maybe I should replace the inversion valve next and see what happens.  Maybe I'll be lucky and my push/pull valve with POP and my QR1 will stop leaking.  I wish I had more information and understood how some of this works better than I do.  What ever happens, I'll keep posting until I get it fixed.  Maybe my drama will help someone else in the future.