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Single line trailer air brakes



I have asked this question previously but didn't precipitate any information regarding the single line air brake; information about modern systems is not relevant.

I have a trailer which is czech army. It has Single Line airbrakes; there is no immediate reason why these shouldn't work as intended with a little attention.

Could any body point me in the direction of some information on this system? Which I assume would be Eastern Bloc. standard for the '60s era.

Anyway as I remember from experiences with a Praga a while back; the service brakes on this worked in a pressure to apply the brakes fashion; that is non fail safe. They did fail regularly due to leaks various.

The trailer has a reservoir in the system; which would suggest brakes would come on when the line pressure in the single line dropped in a fail safe; railway brakes fashion.

However I would like to use the trailer with a tractor with Two Line trailer brakes. Any thoughts on how this might be achieved? This isn't my field really. It is obvious that neither of the lines of the two line set up will alone operate the single line system using the service brake pedal as would be desirable and if I am going to make it work I might as well get it to work properly; having a separate trailer brake valve would be only a halfway improvement.

I have found a couple of companies that seem to offer a "braking system" kit mainly for upgrading agricultural trailers that have hydraulic trailer braking to suit modern tractors that now have air brakes; they also do air brake modules to fit on tractors with only hydraulic trailer braking so there is air braking provided sensed from the hydraulic brake system.

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If this is a straight air brake system (braking application is proportional to pressure in brake cylinder), I would think the only way the trailer brakes could function as service brakes with just a single line would be if there is a separate trailer braking valve on towing vehicle which
line pressure to apply trailer brakes. If not how is reservoir kept maintained at high pressure?


The reduced line pressure is detected by a sensing valve which applies trailer brakes (proportional to pressure drop) from the trailer reservoir (which is fitted with a non-return valve). When pressure in line is restored the valve releases trailer brakes.


The system also functions as an emergency brake should the trailer breakaway and line pressure be lost completely.


If no trailer braking valve on towing vehicle, the towing vehicle would do all the braking and the system would function only as a breakaway emergency brake so long as line was connected.


I guess the trailer reservoir pressure could be initially built up by applying brakes before moving off, the non-return valve would then maintain this pressure until a breakaway when valve would detect loss and apply brakes, and normal trailer braking effected by line pressure - but how does the sensing valve identify between line breakage and zero pressure (no brake force) under normal use? Is it by detecting RATE OF LOSS of pressure?


Edited by N.O.S.
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Just found something which might help you (it was niggling me trying to understand the use of single line!):


Below are three posts copied from http://www.russianmilitarytrucks.com/phpBB3/viewtopic.php?f=10&t=1888&start=40


(Note there seems to be a slight ambiguity about how the system should work, but reckon the answer may be that they were originally emergency brakes only, then later trucks were fitted with more complex brake valves which allowed the same trailer system to function as a service brake)



A) "Air pressure take off connectors on Gaz/Zils etc are just for charging up trailer air tanks. There is no physical connection between operation of the trailer brakes and vehicle brakes. Therefore, the Gaz66 twin servo assembly is just for the vehicle braking system only (not all have twin servos, earlier had single). There is no braking circuit connector for trailer braking, just a air pressure supply from the tank.


Principally when towing a Russian trailer you connect the twist turn air connector, and switch on air tap (under chassis), the vehicles compressor charges the trailer air tank. When you are towing, should your trailer become dis-connected and it attempts to go its own merry way tearing the air pipe. The sudden discharge of air will operate a valve, shutting the sudden escape of air and fully applying brakes on the trailer.


Therefore, my advice, is to be mindful with your overall train weight, these are very stable trucks and there are very few with trailers across here. But, you don't want to be ending up with brake fade on a hill or the tail wagging the dog."

B) "I dunno about the Gaz but all the Zil 131's have a proper single line trailer air brake outlet, this operates much in the way you have described via the use of a relay valve and dedicated air tank located on the trailer, but this wasn't just used as a safety device for breakaway trailers, it was a fully working service brake operated via an additional mechanism inside the Zil's foot brake valve assembly, basically a constant supply of air is supplied to the trailer's air tank until the brake valve on the Zil is operated, when this happens air pressure in the trailer supply line drops, the trailer relay valve senses this pressure drop and switches allowing air pressure from the trailers air tank to operate the trailer's brake actuators, however once the trailer air line pressure is restored the valve switches back and the trailer brakes are now released. This system for trailer brakes has been around for a long while, I'd be surprised if the Gaz wasn't also equipped with the same system as the Zil, it only requires the fitment of a brake foot valve that's capable of controlling the trailer air line, it would be rather strange if this weren't the case as the Gaz would be more than likely expected to pull all the same trailers as the Zil. The easiest way to check would be to undo a union on the trailer brake line of the Gaz, if the air stops hissing out when someone puts their foot on the brake pedal you know it's a working service brake line, not just a air supply line."


A) "Do you know what. Until earlier this year, I thought exactly the same as you. However, we sold a Zil and box trailer a couple of years ago, and only found out 100% when we connected the trailer and wondered why the trailer brakes did not work when you operated the brake. When the engineers looked at the system in detail, we discovered how the system operates. The Soviets weren't bothered if the odd truck ran away.

Also, another reason they use this type of design, as that many Soviet box trailers have a parallel air line and connector running to the rear. There would be another hook and electrical connector. In theory, you are able to connect multiple trailers, and if any came away it would just stop.

However, generally military convoys could be a mile long and were quite slow. "

Edited by N.O.S.
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Thanks N.O.S. that is most helpful.

The trailer is indeed fitted with complete rear drawgear, air pipes through and electrics for running as a train.

I haven't got this system to operate as there is several leaks in the rubber sections of pipework but seems to be the best plan to do away with it and set it up with a conventional system made up with some readily available components.

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