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Author Topic: Air Brakes - Application Pressure? Related to toad brakes  (Read 9073 times)
Sean
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« Reply #30 on: December 10, 2008, 10:15:48 PM »

Jim,

After I just posted my last reply (wherein I discussed two-line, breakaway-protected systems as found on trucks), it now occurs to me to ask:

How are you implementing automatic application of the toad brakes in the event of a break-away, as required by many states?

-Sean
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« Reply #31 on: December 11, 2008, 06:13:41 AM »

I have read this thread with interest and am wondering why not use a system similar to trailer brakes such as used on car trailers/tow dollies.  They have a kind of spring loaded torque rod and as the towing vehicle slows the brakes are applied on the trailer.  Using this technique in conjunction with air (or other) assisted braking would seem to be the best way to keep the load off the front vehicle.  This would also reduce stress on the tow/hitch set up if properly adjusted.  I mention this as the brake air line pressure is going to vary so much it would seem wiser to have a separate system based solely on deceleration.
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rv_safetyman
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« Reply #32 on: December 11, 2008, 08:02:04 AM »

Sean, I have not had time to look for the "excess flow" valve, but I will do that in the next few days.

I have not yet implemented a breakaway system yet.  Jack Conrad and another person have been giving me guidance on how to do that. 

Basically, Jack uses a system the mimics the M&G system.  That is to use a typical trailer breakaway switch that operates a solenoid valve.  I have been looking for a suitable pressure reservoir.  One person suggested using PVC.  Not sure I feel comfortable with that material.  I do have a copper tubing accumulator that I built for another application that I will probably use.

The system seems pretty simple.  Air in to a pressure chamber via a one way valve, air out the the brake cylinder via a solenoid activated by the breakaway switch.  I have to think my way through how to deal with stopping the flow back out the broken application line.  I think that can be done with another check valve.  Jack has given me that information, but I have not studied it in detail.

Jack charges his pressure cylinder via the air from the brake supply line.  He hits the brakes with full application and that charges the cylinder.  I am a bit concerned about that process on my application, as you need to bleed the vacuum out of the toad brakes before you make the full pressure application.  Otherwise, you could overload the the master cylinder.  That is not an issue with the M&G system since it is between the vacuum chamber and the master cylinder. 

When we get ready to depart, I have my wife apply the brakes a few times while I check the toad.  When I get the breakaway plumbed in, I will have her do several light applications while I do my checking (to bleed the vacuum) and then I will do a couple of full applications before I depart.

At least that is the plan

Jack, my apologies to you.  I probably butchered you "system", but I know you will straighten me out Smiley.  I think there is a thread on this specific subject, but I did not do a search.

zubzub, what you are describing is what is typically called a surge brake system.  It is used on tag type trailers (and I think one dolly manufacturer uses it).  The tongue is free to float and activate a master cylinder if the towing vehicle slows down.  The only ones I have seen are a "closed" hydraulic system contained withing the trailer.  I am not sure how you could make that concept work with a air system or withing the hydraulic system of the toad.  You would definitely not want to try to plumb it into the toad hydraulic system!!!

Many folks do not like trailers with surge brakes because they activate when you are trying to back the trailer.  That would probably not be an issue with a bus/toad, since you really should not try to back them up (exception very slow short distances).

Jim
« Last Edit: December 11, 2008, 08:11:46 AM by rv_safetyman » Logged

Jim Shepherd
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« Reply #33 on: December 11, 2008, 03:07:20 PM »

I think the over-riding issue with surge brakes is that there is no way for the driver to confirm that the brakes on the trailer are actually working, and to what degree.

In other words, how do I test them during the pre-flight?

happy coaching!
buswarrior
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JackConrad
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« Reply #34 on: December 11, 2008, 03:59:22 PM »

Jim,
    I will try to explain my break away system. I installed a small aluminum pressiure tank on the toad. It is fed through a one way check valve from the tag axle brake line. (same line that activates the M&G toad brake). Whyen doing our pre-trip, I make a hard service brake applicatio (at least 60 PSI). This fills the pressure tank and a one way check valve prevnts the air from leaving the tank.  A tee is installed between the check valve and the tank.  This goes to a 12 volt solenoid controlled valve. When the breakaway switch is activated it supplies power to the solenoid which opens the valve between the pressure tank and the M&G, and simultaneously closes the valve between the M&G and the airline from the bus.  Jack
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« Reply #35 on: December 11, 2008, 07:17:36 PM »

Hi Jack...what control the air leaking from the bus's brake service air during brake application after the line break away from bus?

Thank you. Gerald
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« Reply #36 on: December 12, 2008, 05:15:42 AM »

Only leaks when service brakes are applied. I did a test drive with an open male quick connect couping installed in bus fittng. I had plenty of air to stop (both normal and "emerency" hard stop). Once stopped, I set the park brake and released the service brake (no more leak). Dash air pressure gauge never showed less than 80 PSI.  Normal stopping uses about 10-15 PSI, hard stop used about 40 PSI, so in either stop, you are not pushing air out the leak at anywhere near the tank pressure of 125 PSI.  If toad should break loose, only bus brake application will be to stop the bus as quickly as possible and set parking brake. This should not be a problem.
   Now if I was coming down a mountain at 60 MPH, I migh have a problem, but first, we don't travel in big mountains and if we did, I don't expect to come down at much faster than 30-40 MPH.  Just our way, YMMV  Jack
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