Simultaneous application of the service brake and the spring parking brake

[Barn Owl]

I didn't know that simultaneous application of the service brake and the parking brake (spring brake) was something one shouldn't do. I needed to replace my air relay valve and while doing the research I kept on seeing anti-compounding valves mentioned in addition to the regular valves. While reading through the Bendix web site I read that applying both the service brake and the parking brake at the same time (without an anti-compounding valve) could damage the brake components. I had never read or heard anything about this before so I thought I would bring it up here. The fleet mechanics where I work know about it and say they have seen the damage it can cause. I guess I am lucky because I have applied full brake pressure while the spring brakes were set and the best I can tell I didn't break anything. The anti-compounding valve eliminates this potential problem but I do not have an anti-compounding valve on my bus. I will just be more careful in the future. Does anyone else know about this?

[47FLXclipper]

I believe it's really only a problem with systems that have automatic slack adjusting

Bill

[JackConrad]

    I think applying both parking and service brake at the same time can apply more pressure to the brake system than it was designed for. This coulg result in bent or broken components
    I think DD3 applies 85 psi through park circuit. 85 PSI x 30 sq. in. diaphragm=2550 PSI. A firm service brake application of 60 PSI x 30 sq.in. diaphragm=1800 PSI.  2550 + 1800 =4350 PSI applied to brake components. If my thinking is incorrect (would not be the first time), please explain correct answer.  Jack

[Sojourner]

Amen to Jack's post as far being said about 2 diaphragms being applied at the same time. Over stressing the can's mounting bracket too many times will cause to crack & broken. Have someone apply both service while you watch it flexing. In other words, it like having 1 extra brake can to 1 wheel brake cam.

If on level or steep grade, always release service pedal first then quickly apply parking brake.

This is also apply to spring brake system.

FWIW

Sojourn for Christ, Jerry

[HighTechRedneck]

I believe it's really only a problem with systems that have automatic slack adjusting

Bill

Anti-Compounding Brake Theory

This concludes the anti-compounding description. By controlling the forces being applied to the slack adjusters, we have reduced the stress placed on the slack adjuster parts, and we have also eliminated the potential of overflexing the automatic slack adjusters enough to the point of over adjustment by the automatic slack adjusters.

Before anti-compounding valves, automatic slack adjusters could be flexed by the compounding forces upon the automatic slack adjuster, and this would allow the automatic adjuster to catch the next adjustment notch, even though it wasn't really needed. This resulted in automatic slack adjuster over-tightening, which was a real problem, until anti-compounding valves came to the rescue.

[Dallas]

    I think applying both parking and service brake at the same time can apply more pressure to the brake system than it was designed for. This coulg result in bent or broken components
    I think DD3 applies 85 psi through park circuit. 85 PSI x 30 sq. in. diaphragm=2550 PSI. A firm service brake application of 60 PSI x 30 sq.in. diaphragm=1800 PSI.  2550 + 1800 =4350 PSI applied to brake components. If my thinking is incorrect (would not be the first time), please explain correct answer.  Jack

Jack, I'm not saying you are wrong in any way shape or form, I just wanted to point out that with my trucks, and with my PD4103, I have a dual needle brake application gauge. When ever I applied my service brakes I could actually see how much pressure was going to the brake system, both front and rear. If, and it's an important "IF", the application gauge needle ever exceeded 15 psi on a pedal application, I knew it was past time to adjust the slack adjusters.

On another note, during roadside DOT inspections, the officers will do a air pressure brake application leak down test. This requires a full brake application, the pass/fail point was an air loss of 4 lbs in 1 minute, engine off. If you failed, you were red tagged until such time as the leak was fixed and reinspected by a qualified DOT officer.

I have had a major brake failure before on a downhill run with 122,000 pounds on a RGN Beam trailer, believe me, if I were going to blow a brake diaphragm I would have at that time.

Dallas

[Dallas]

Another excellent reason to get rid of the automatic slack adjusters and only use manual adjusters. I've seen automatics tighten up too many times for no particular reason whatsoever.

There is absolutely no reason in my mind that a person who is interested in safety to NOT crawl under and check the slack adjusters. Automatic Slack adjusters are not fool proof, there are many reasons and ways for them to fail.

Just my pair O' ¢'s

Dallas

I believe it's really only a problem with systems that have automatic slack adjusting

Bill

Anti-Compounding Brake Theory

This concludes the anti-compounding description. By controlling the forces being applied to the slack adjusters, we have reduced the stress placed on the slack adjuster parts, and we have also eliminated the potential of overflexing the automatic slack adjusters enough to the point of over adjustment by the automatic slack adjusters.

Before anti-compounding valves, automatic slack adjusters could be flexed by the compounding forces upon the automatic slack adjuster, and this would allow the automatic adjuster to catch the next adjustment notch, even though it wasn't really needed. This resulted in automatic slack adjuster over-tightening, which was a real problem, until anti-compounding valves came to the rescue.

[Barn Owl]

I believe that automatic slack adjusters get the worst of it from what I have read, but according to the fleet mechanics where I work, they have seen bent/broken mounts, stripped splines and other damaged components. From what I have learned it's not a good idea, no matter what type of adjusters one has, unless there is an anti-compounding valve installed to protect the system.

[JackConrad]

Dasllas,
   I agree that normal service brake applications seldom exceed 15 PSI ( I have a brake application pressure gauge also). I can get 75-80 PSI with a firm brake application when sitting still. I usually do this AFTER releasing the parking brake to allow the DD3 to release.  Jack

[cody]

Ok guys, now you've got me corn-fused again, to apply my parking brake I have to press down on the service brake pedal and pull the knob, to release it I have to again press the brake down and push the knob in, mine won't activate without pressing the brake pedal, am I doing something wrong or do I have a malfunction?  Expiring minds need to know.

[Dallas]

Ok guys, now you've got me corn-fused again, to apply my parking brake I have to press down on the service brake pedal and pull the knob, to release it I have to again press the brake down and push the knob in, mine won't activate without pressing the brake pedal, am I doing something wrong or do I have a malfunction?  Expiring minds need to know.

Cody,

You have DD3 brakes, not spring brakes.
Spring brakes are a completely different kinda critter than the DD3's which are common on many buses.

Dallas

[cody]

Thanks dallas, I was feeling ever so spethal, now I'm back to being common lol.

[Lin]

I was once warned about this double brake application with my other bus that had spring brakes.  I did not think it was a concern with my DD3 brakes now.  Am I mistaken?  Do I still have to watch for double braking?

[RJ]

Ok guys, now you've got me corn-fused again, to apply my parking brake I have to press down on the service brake pedal and pull the knob, to release it I have to again press the brake down and push the knob in, mine won't activate without pressing the brake pedal, am I doing something wrong or do I have a malfunction?  Expiring minds need to know.

Cody - and others with DD3 brakes -

With the DD3s on your coach, to apply the parking brake, all you should do is maintain a LIGHT service brake application - just enough to keep the coach from rolling, then pull the parking brake knob up to apply.

To release the parking brake:

1. Make sure air pressure is up all the way (governor has cut-out)
2. Cover the service brake with your foot - basically just resting on the pedal.
3. Push in the Parking Brake button/knob
4. Make a FULL service brake application and hold it for 3 - 5 seconds.
5. Release the service brake, and away you go.

If the Parking Brake does NOT release after the above four steps, then repeat the procedure.

If it does not release after the second run-through, it's time to have the system checked by a competent bus technician who's familiar with DD3s.

FWIW & HTH. . .

[buswarrior]

Hello.

Thanks Barn Owl, for asking this over here...

In this chapter:

anti-compounding history
auto vs manual slacks
compounding a DD3 and more...

There is a great amount of confusion regarding anti-compounding. In looking for answers over the years, I have so far been unable to get some verifiable time frames as to when air systems were plumbed in this self destructive way, and at what point this stopped.

I am led to believe that it was for a relatively short period back in antiquity, with possible exceptions for buyer specifications, and the complete sky is the limit of what a mechanic did to the plumbing during the vehicle's life...

And that once latched on to an idea, generations of professional drivers keep passing down to the next generation the things they know, without any way for the older, irrelevant info to be stopped.

A driver needs not worry about combining a simple foot brake application with the spring brakes. By simple, I mean the kind of pressure that it takes to keep the vehicle from rolling around. That combination of forces is still far less than a regular full service application. The springs are at best only as strong as 60  lbs of air pressure.

A full service brake application, without an anti-compounding feature would put a lot of stress on the mechanical brake components. Perhaps much of the hysteria has been generated on purpose by those who employ drivers, and wanted to ram it home in their heads that economic sanctions would be placed on their backsides if they broke the brakes. No explanations, just a threat. Anyway, it seems to have worked, as we're still wondering about it all...

The technical explanation at the link that HighTechRedneck has in his post is as good an explanation as any,(go give it a read!) until it combines the concepts of anti-compounding and auto-slacks. As best as I've been able to determine, anti-compounding stopped being a problem many, many decades ago, a time when there were very few auto-slacks, if any, available on the market. I wouldn't have included that combination of thoughts, but that's just me.

As an aside, auto-slacks have gotten a bad reputation over the years for only one reason: lack of proper maintenance, in particular, regular greasing, being adjusted manually, and improper install. That being said, the relatively small mileage done by a busnut, and the resulting small brake wear, the auto slack may go for years without advancing. Manual slack adjusters might be preferable in order to defend against the problems associated with lack of activation. Don't rush out to change yours until you have run out of money to spend on other things?

Regardless of adjuster type, I hope we are all going under the bus, having an assistant apply and release the brakes in order that we may measure the applied pushrod stroke of the chambers at some periodic interval, and ensuring that they are not overstroking?

Have no fear, DD3 owners, you cannot compound a DD3!

If you have a cutaway drawing of the valving and a schematic, (from my handout package at Arcadia a couple of years ago, or your MCI manual) note the exhaust plumbing, and the position of the valving internals. Note the exhaust from the driver's parking control knob is directed into the service brake circuit, in order to exhaust through the foot pedal. The air pressure from a service brake application will be directed into the exhaust from the driver's control push/pull valve, thereby pressurizing the same plumbing as if you had released the parking circuit yourself, and releases the parking application "automatically", and only for as long as you make that service application. Removing your foot from the brake pedal, the valving will reset the parking brake.

If you still have your parking brake indicator in the dashboard, it will go out if you stand on the brake pedal while the parking circuit is applied.

Hmmmm, so if this is the case, and all my valving is working as designed (not sticking or otherwise misbehaving, as age and lack of use will cause, and the air regulator for the parking circuit is properly set..) then what might be the effect of the discouraged heavy service brake application prior to parking circuit application?

Not as bad as first imagined.  You may get a stronger set of the brakes, requiring the proper methods for release as detailed by RJ, (which we may be getting away with not doing, because the parking circuit has not set as strongly the way we have been parking usually. This can be alarming to the driver, further fueling the misinformation) but the full service application in a properly working system will more than overcome the earlier explained blended application that resulted.

Amazing the amount of thought that goes into the brake plumbing!

happy coaching!
buswarrior