Hello everyone,

I'm doing some research into a case which involves a truck that rolled backward some distance down a slight slope due to what we assume to be an inadvertent disengagement of the Maxi/parking brake (spring brake). I have read some literature on truck brake mechanics and would appreciate your expertise.

The truck in question is a 1994 Ford Aeromax L 9000 prime mover and had tandem trailer attached (but unloaded) at the time.

I would be grateful if anyone could shed any light on their own experience or a story they have heard of similar incidents of Maxi brakes popping off.

Also, i have a number of questions regarding the brake systems on such a truck:

1. When you engage the Maxi brake, does this engage all the brakes on the prime mover as well as trailer OR just on the prime mover?

2. In the Ford L9000, there is a separate stalk lever on the right of the steering wheel to activate the trailer break. What is the main purpose of the trailer break and when would you use it?

3. What is the difference between the Maxi brake and trailer break in terms of the way they work and purpose (if any, besides one is obviously for parking and other to retard the trailer)?

Your expertise and comments on any of the above would be greatly appreciated.

Alternatively, since this research does involve some degree of confidentiality, you may contact me directly at my company email at [email protected].


Regards,
Michael Wang

 

quote="schmichael"]Hello everyone,

I'm doing some research into a case which involves a truck that rolled backward some distance down a slight slope due to what we assume to be an inadvertent disengagement of the Maxi/parking brake (spring brake). I have read some literature on truck brake mechanics and would appreciate your expertise.

The truck in question is a 1994 Ford Aeromax L 9000 prime mover and had tandem trailer attached (but unloaded) at the time.

I would be grateful if anyone could shed any light on their own experience or a story they have heard of similar incidents of Maxi brakes popping off.

"Maxi brakes", or spring parking brakes do not pop of without reason... you should first be determining brake pushrod stroke (adjustment), and the chambers for broken springs... that could sum up the whole incident

if the brake foundation components are basically there, the brakes are adjusted, and the springs are intact... the vehicle should not be able to move....... most importantly that the brake pushrods are not overstroked

Also, i have a number of questions regarding the brake systems on such a truck:

1. When you engage the Maxi brake, does this engage all the brakes on the prime mover as well as trailer OR just on the prime mover?

It can... depending on the type of park control valve

generally speaking without much specific vehicle info; when the yellow dash valve is pulled to apply tractor brakes, the trailer button pops as well... not that it would matter, as the tractor park brake has lots of strength to hold an unloaded trailer

2. In the Ford L9000, there is a separate stalk lever on the right of the steering wheel to activate the trailer break. What is the main purpose of the trailer break and when would you use it?

that is the spike, or trailer service brake valve... it applies service brakes on the trailer only

3. What is the difference between the Maxi brake and trailer break in terms of the way they work and purpose (if any, besides one is obviously for parking and other to retard the trailer)?

the spike is strictly not for parking, again it controls trailer service brakes only, and they will not hold the vehicle without sufficient air reservoir pressure

Your expertise and comments on any of the above would be greatly appreciated.

Alternatively, since this research does involve some degree of confidentiality, you may contact me directly at my company email at [email protected].

again, this is all based on the small bit of info you have provided

Regards,
Michael Wang[/quote]

 

bob h:
Thanks for the reply.

1. In the scenario I'm dealing with, there is definitely no fault in the adjustment or spring mechanism of the brake. All that has occurred is the Maxi brake knob has popped out by itself, possibly due to it not being pushed in fully or a number of other factors yet to be determined.

2. I'm was asking in relation to parking brakes in general, ie. when you push the knob in, does this engage the brakes on the trailer as well? Or does this depend on the specific manufacturer's configuration?

3. I read that the trailer brake is commonly applied in situations to prevent the trailer from fishtailing or jack-knifing. Also, in the Ford Aeromax, the spike is spring loaded so that it "springs" back to the off position as soon as the operator lets go of the spike (hence unsuitable for parking). Is this the usual configuration?

I guess in relation to 2. i mean does pushing the parking brake knob also brake the trailer as if the spike was being held down constantly?

Cheers,
Michael

 

quote="schmichael"]bob h:
Thanks for the reply.

1. In the scenario I'm dealing with, there is definitely no fault in the adjustment or spring mechanism of the brake. All that has occurred is the Maxi brake knob has popped out by itself, possibly due to it not being pushed in fully or a number of other factors yet to be determined.

the button is pushed IN to release the park brake.

when the button "pops out", it is either because; the driver has pulled it out or the air pressure in the tanks has dropped low enough to cause automatic application, in either cases this APPLIES the park brake

2. I'm was asking in relation to parking brakes in general, ie. when you push the knob in, does this engage the brakes on the trailer as well? Or does this depend on the specific manufacturer's configuration?

if both buttons are IN (park brakes released... tractor & trailer), then you pull the yellow button OUT to apply the park brakes, the most common dash valves/modules would cause the trailer brakes to apply as well...

but, again the tractor brakes are fully capable of holding both units (even if the trailer brakes were not applied at all). alternatively, the trailer brakes would likely have no issues with holding the combination as well

when trucks "roll away" it is often caused by the valve not being pulled at all (....my bad?). if foundation brakes, adjustment, and chamber springs are all sound, then the vehicle most likely rolled due to lack of park brake application

3. I read that the trailer brake is commonly applied in situations to prevent the trailer from fishtailing or jack-knifing.

no, how is applying the trailer brakes going to prevent fishtailing... jack-knifing?!?!?!?

Also, in the Ford Aeromax, the spike is spring loaded so that it "springs" back to the off position as soon as the operator lets go of the spike (hence unsuitable for parking). Is this the usual configuration?

most are

I guess in relation to 2. i mean does pushing the parking brake knob also brake the trailer as if the spike was being held down constantly?

re-read the explanation of how the brakes work above ^

Cheers,
Michael[/quote]

 

Guess this Furd is only a single axle?

Be sure that the slack adjuster is at 90 degrees to the brake cannister rod. This provides maximum braking with the service and parking brakes.

That parking brake lever is used mostly to test the coupling between the trailer and tractor after connecting.

 

I'm with bob, broken springs. Both have to be broken for this to happen. Sounds like the vehicle was not properly maintained. I reserve judgement though, as I do not know all the facts.

 

As mentioned previously the knob pops out to apply the brakes. It is physically impossible for it to inadvertently release the brakes without significant manual force on the knob. That being said a number of things could be the case:

a. The knob could be bumped by somebody's knee or elbow grabbing around in the cab for something. Human error in other words

b. The yellow (tractor) knob can be pushed in and stay in without the truck rolling if it was on a very slight slope, rock under a tire, or something, but then after someone walks away whatever was holding the truck finally sinks into the mud or something and the truck rolls away. This happens fairly frequently again human error

c. The yellow knob could be pushed in and stay in when the truck air pressure was low and building up (engine running) while the system still did not have enough air to physically release the brakes, as air pressure increased with the truck unattended, the truck could then roll away

d. The red knob could have been the only one pulled with a "park on air" type trailer (no spring brakes). After the air in the trailer leaked away and the tractor brakes still were released, the truck could roll

e. I am sure there are other scenarios I have not thought of just now

On the Bendix MV-series trailer control dual function valves pulling the yellow knob pops the red one also, as well as some Midland models that are similar. If it is two separate valves then pulling the yellow knob is also supposed to apply the red one but there might be a third (blue) valve that allows the trailer to be released again for air charging, and any number of configurations correct or otherwise could be possible depending on what hillbilly has had his hands in there replumbing things.

The spike is always "supposed" to be spring loaded but often (usually?) the spring is so weak it does not return all the way so there is always the danger of someone using it for parking and thinking it will hold, then either having the spike return to the released position slowly or air leak away.

No it works in a totally different manner. The yellow and red push pull (emergency/park) knobs work by supplying air to release brakes and exhausting air to apply brakes. The service brakes (foot treadle/spike) work by supplying air to service brakes to apply them and exhausting air to release them.

Like I alluded to above, old trailers often have one air reservoir and one multi function valve and no spring brake, and apply reservoir pressure to the trailer brakes when emergency line pressure is lost (inversion feature), but only as long as the air on the trailer does not leak away. This can cause all kinds of pitfalls to one who is not aware of the quirks

---

If all this makes no sense to you you might try some of Bendix's training materials, they are very nice, and Paul Jones at Bendix understands the ins and outs of this stuff well too, as well as their phone number 1-800-AIR-BRAKE

Birken

 

Thanks for all the replies everyone. That's a lot of info for me to mull over. I may have a few more questions after I look into those things you mentioned.

Cheers

 

On the dash of a truck you have two knobs. A yellow and a red. The yellow know controls the trucks parking brakes and the red one the trailer parking brakes and air supply. When the knob is pushed in it releases the parking brakes, when you pull it out it engages them. If the brakes are properly adjusted and the spring canisters are in working order the parking brakes are able to hold a fully loaded truck (80,000 lbs) on a steep hill.

The trailer brake lever (spike), located either on the steering column or on the dash often will not stay down for very long and will return to the "off" position. This lever only activates the service "air" brakes on the trailer. Many times if a driver uses this lever to try to "park" the truck, the truck will end up rolling. Also every truck I have ever been in has had a warning sticker stating not to use this lever for parking.

Some old timers believe the use of the braking lever can help correct a jack knife situation by slowing down the trailer and allowing it to return to tracking behind the tractor. The reality is that more often than not it will increase the jack knife because it often causes the trailer wheels to lock up resulting in an uncontrolled slide.