DD3 Brake Test Question

Lin · February 12, 2017, 03:41:28 PM

I decided to test my brakes today and was unsure about a couple of things. I did a one minute leak down test at 120#, and the needle barely budged. However, if I let the air drop down to below 40#, the needle would very slowly decline with the same brake depression. That seemed like it would be reasonable to me but wanted to ask to be sure.

Another things is that I see that the spring brake test calls for feathering the air pressure down to see if the parking/emergency knob pops out at between 20# and 45#. Is that the same on DD3's; should the knob pop out on its own? Mine did not. Thanks

Dave5Cs · February 12, 2017, 09:12:15 PM

Lin I do mine at 100lbs and then pump it down to 80 and wait for it to build back up to 100 lbs in about 30 seconds to 45 seconds.

Lin · February 12, 2017, 10:21:08 PM

Dave--What rpm do you run to check it from 80 to 100#? If you continue to pump the brakes down to 20# with the parking brake off, does the parking brake knob pop out on its own as it does with spring brakes?

bevans6 · February 13, 2017, 03:42:23 AM

When I took my mandatory air brake school these things were taught and we had to write a test every two years. The air loss test is only valid at over 100 psi, and is in two stages. One is just sitting there, loss of 3 psi per minute or less, the next is exactly the same with the brakes fully applied. The compressor recovery test was from 85 psi to 100 psi in a certain amount of time, it varies by state and province it seems, and the rpm was specified at 1,000 rpm. My bus typically recovers in around 18 seconds. On my MCI the push pull valve is set to pop automatically around 30 psi but as it's fed from the emergency brake tank (which is protected from air loss in the main system by check valves) you need to test it by draining the all the other tanks first, then draining the emergency tank. I have no pressure gauge on the emergency tank so I just drain it and then check if the valve popped at some point. Just fanning down the service brakes will never make it pop unless there is a fault in the emergency tank system. This creates confusion at annual inspection time.

If you drain a typical bus system to 40 psi, different things will happen depending on where your pressure gauge is fed from and the health of the rest of your system. On my 1980 MCI the pressure gauge is fed from the service tank ( the main dry tank for the brake system). If I pump down to 40 psi air will slowly backfill from the auxiliary tank through the pressure protection valve, which seems to not work as well backwards as forwards (it usually is expected to keep air out of the auxiliary system when the brakes are initially filling and in the case of a failure in the auxiliary and suspension systems, so it closes if the pressure in the service tank is less than 60 psi).

luvrbus · February 13, 2017, 04:13:44 AM

Lin,your bus is a single air system it will not work the same as Brian's and Dave's dual air system there are no tests even for a single air system it seems like your is working fine fwiw any bus made after 1975 had a dual air system

Lin · February 13, 2017, 09:06:38 AM

To be clear about DD3 emergency function. It seems that whereas spring brakes lock automatically when there is an air loss and the valve pops out,the DD3 brake requires a service brake application for them to lock and the valve does not pop on its own. Does that make sense?

luvrbus · February 13, 2017, 10:06:16 AM

Quote from: Lin on February 13, 2017, 09:06:38 AM
To be clear about DD3 emergency function. It seems that whereas spring brakes lock automatically when there is an air loss and the valve pops out,the DD3 brake requires a service brake application for them to lock and the valve does not pop on its own. Does that make sense?

No the valve should pop automatically at 40 PSI it does on the dual air setup with the DD-3 brakes

buswarrior · February 13, 2017, 10:57:55 AM

A little knowledge is a dangerous thing...

The pluming and function of spring and DD3 systems is different.

It is best to forget everything you know about spring brake circuits, they just confuse things when dealing with DD3.

We also have the clear and present danger of pre and post 1975 FMVSS being mixed together, and lingering old State regulations still influencing brake plumbing.

For example, there are 8 different air diagrams for the MC8, depending on which regulations it was built to.

We can't speak about DD3 brake details for one coach, and expect it to be correct for another!!!

A great brake system, but one needs to know about their own, and know that other vintages are different.

A very long, multi-chapter book would be fun to write... who's paying me?

happy coaching!
buswarrior

luvrbus · February 13, 2017, 11:08:32 AM

LOL I was waiting for BW to come in and do the dirty work he doesn't piss people off as bad as I do

Lin · February 13, 2017, 02:07:36 PM

Well, my valve did not pop at 40#. I will try it again though.I will also try and research old systems. So far, I have not hit on any good sites though.

luvrbus · February 13, 2017, 03:36:01 PM

Quote from: Lin on February 13, 2017, 02:07:36 PM
Well, my valve did not pop at 40#. I will try it again though.I will also try and research old systems. So far, I have not hit on any good sites though.

It should tell you in the manual in the brake section if you have it

Lin · February 13, 2017, 03:43:30 PM

Right, when all else fails, read the manual!

Curses! The Maintenance Manual says nothing about it.

buswarrior · February 13, 2017, 10:57:47 PM

The "popping parking control brake valve" isn't really much of test. Ontario dropped it from the air brake program almost 20 years ago. Waste of time.

In a spring brake system, during a real air loss situation, the vehicle would have already stopped by itself before the control valve pops, since the big springs start to apply as the air drops through 60 lbs, and are already quite firm as the pressure drops through 50 lbs.

The control valve may "pop" anywhere from 25-40 lbs.

The "popping" just proves the little spring in the valve is doing something, which has no bearing on its functionality, you could remove the spring and still get the valve to function. And, it is quite rare for these to fail, air leaks from old age and breaking the finger tabs off the plunger, being the common issues.

As for DD3 research sources, busnuts have two. Between Brian Evans and this faithful scribe, on BCM or over on BNO, we've written most of it a few times, scattered through the years. The search function brings it up for your viewing pleasure.

With the exception of a couple of recent MCI pages, and threading together old Bendix stuff (it may not even be out there anymore?)
Info has always been hard to find on the DD3 system.

happy coaching!
buswarrior

chessie4905 · February 14, 2017, 04:00:05 AM

Bendix makes a comment about pop out pressure on page 62.

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.suspensionspecialists.com/techinfo/Bendix_Brake_Handbook_2009.pdf&ved=0ahUKEwj4sv2Tw4_SAhVC1CYKHfC8BHkQFghZMA4&usg=AFQjCNGnaB2Zcot0loJxuqbIequ9Xx1SWA

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.smartdrivetest.com/air-brakes/applied-stroke-measurement&ved=0ahUKEwjjk5aOyI_SAhVE5SYKHd9WBsgQFgh2MBI&usg=AFQjCNGskJMZ6QHMNdUCQ31jpaGGFBzMwg

Appendix B has a little info on burnishing new brakes. An IR heat gun could be used to check drum temperatures. Everyone aught to have one of these guns now that they are reasonably cheap. Good for checking brake drums, radiators, tires, batteries, power cords, especially plugs and sockets, and many other uses. With some practice, can be used to predict when wife is about to blow her top and lower crisis temp from Defcon 5 down to 3 or 2. Most coach owners wife's seldom see Defcon 1. 😉