Any members with an intimate knowledge of HGV air brake systems?

Chris Wilson

Chris Wilson

Chris Wilson

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Joined
28 Nov 2003
Posts
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Location
Shropshire
I have had what i believe is a problem with a leaking to atmosphere unloader valve, a Bosch one. I bought a new one off Ebay (OK, OK, I know..... ;)), and now it's here it has one less port. I need to know if this variation can be worked around as I need to move the truck and the brakes are locked on, and even if I wind them off manually it won't make enough air with the leak to work the pneumatic gear change or clutch assist.... Thanks.

The added complexity is it's Demon Tweaks' old transporter, custom built from a Mercedes artic tractor unit with an extended chassis and a four car box with a two stage tail lift. So the original braking system expected an artic trailer... As you can tell I know next to sod all about truck air brake systems save there's a LOT of pipes, valves and reservoirs <LOL>

Happy Christmas and a healthy and prosperous New Year to all.
 
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kink the pipe before where the air leak is and cable tie it or use vise grips to temp stop the leak and build up the air to move it

**** knows about the valve, sorry!
 
builder22 said:
kink the pipe before where the air leak is and cable tie it or use vise grips to temp stop the leak and build up the air to move it

**** knows about the valve, sorry!
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Problem is the unloader valve is right after the compressor and I don't fancy blocking off the unloader valve as it may well act as the only safety "blow off" valve. Good idea though, but I am fearful of the consequences :) Thanks.
 
nlel1975 said:
I doubt you'll find anyone in here with this kind of knowledge, most of us are just amateur's.
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Oh I dunno, when I had a question about my 2 post vehicle lift a very helpful chap on these forums came along and told me in depth how to check for wear in the nuts that raise the lifting arms. There's a wealth of hidden talent about :)
 
Stu999 said:
@Scania probably
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I drive - and break - them, not fix them! :D

@Chris Wilson - Is this the load sensing valve? which serves the purpose of automatically adapting the brake pressure and therefore the brake force according to the load (weight) of the truck?

It is compulsory for anything over 7.5t Gvw, I’d be inclined to think your right in assuming the braking system is set up for a trailer combination….

I’d suggest you ask on somewhere like Trucknet or indeed consult an expert, you said it yourself that you “know next to sod all about truck air brake systems” if this is the case, I’d get some expert advice before proceeding.
 
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Scania said:
I drive - and break - them, not fix them! :D

@Chris Wilson - Is this the load sensing valve? which serves the purpose of automatically adapting the brake pressure and therefore the brake force according to the load (weight) of the truck?

It is compulsory for anything over 7.5t Gvw, I’d be inclined to think your right in assuming the braking system is set up for a trailer combination….

I’d suggest you ask on somewhere like Trucknet or indeed consult an expert, you said it yourself that you “know next to sod all about truck air brake systems” if this is the case, I’d get some expert advice before proceeding.
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No it's a complicated looking valve that allows the compressor to build the full air pressure in the tanks, then vents further air to atmo'. When the pressure drops below whatever threshold it's set to, it closes again and the compressor recharges the air storage tanks. It also feeds the air dryer, and "other stuff" :) It's a mess of pipes and unions and probably Ebay was not the best place to buy a new one, but Mercedes were talking something like £750 for one and I wasn't even 100% sure the fault lay with this thing.
 
MrKnifey said:
Do you have a photo of the valve.. 11 years as a HGV mechanic, so maybe able to rejog my memory with a photo or two
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I am in the embarassing position of having to admit that I can't find either the old one I took off, nor the new one. I remember putting them somewhere safe, but obviously in hindsight, too bleedin' safe... I am blaming the beer for now, once I lay hands on them I will, with great thanks, post photos. I appreciate the offer of help!
 
Does the pipework off the valve feed the front and rear brake tanks etc? If so are you talking about what we call the four way valve?
 
MrKnifey said:
Does the pipework off the valve feed the front and rear brake tanks etc? If so are you talking about what we call the four way valve?
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Yes, I think it does, via a drier for the air supply. I'll have a proper look for the units tomorrow. It's hard to trace the pipes as it has full length belly lockers and getting under it with it on the deck is difficult.

I just found the handbook, I think it's possible to mechanically get it in a forward or reverse gear without air, but it'd be a hell of a lot easier if the all the air stuff worked again :) Of course it has a pool of water under it at the moment too!


EDIT

No it's definitely not the "4 way valve", I just Googled them, it's very similar to this one, but not the exact model:

www.airbrakedirect.com

9753034730

Port Thread 1-2: M16 x 1.5 </br> Port thread 1: M22 x 1.5 </br> Port thread 21: M22 x 1.5 </br> Port thread 22: M12 x 1.5 </br> Port thread 3: Union Ø 20 mm </br> Port threads: M22 x 1.5 </br> Type: With one way valve </br> Version: With tire inflating va
www.airbrakedirect.com
 
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MrKnifey said:
You could T off one of the outlets to feed the pipework left without a supply.
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I could maybe connect up an air bottle (I have diving bottles with pressure regulators and a diving bottle refilling compressor for air starters on Formula race cars), so have "portable" air. But don't have any fittings of the sorts of sizes on a truck. I have loads of small stuff for the ultra unreliable air suspension on Merc cars, 4 x 4's and Range Rovers, but that's relatively tiny stuff :( Where there's a will there's a way though, I appreciate your time!
 
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If I remember rightly putting something down in a safe place you will find it's right in front of your nose -With me that's usually the last place I look. :D
 
For completeness and with thanks to the people who replied, I found the valves, it was the compressor unloader valve that controls the whole sytems air pressure. Replicating the missing port on the new but slightly different valve was possible, the rudiments of an undrilled and untapped port were there in the casting, but it would have been tricky and time consuming.

So I managed to free the corroded bolts in the old one and strip it down. It had stuck internally with dirt and minor corrosion, but the diaphragm was fine and it went back together alright with new fasteners and it worked properly immediately!

I then fixed the extremely complicated air suspension and adjustment for the driver's seat that was bleeding off air. That fix was easy, I capped the main air feed off to it and left its problems for a later time <lol>

So I now have 16 gears, an assisted clutch and operative brakes again, and moved it out of the way and back again, all of twenty yards...

Thanks again!

Oh, and the fittings for injecting an external air supply are called "test fittings", and cost £3 each so I got two, they made finding the air leak to the seat a breeze as you could hear and trace it easily without the engine running, by supplying air from a compressor or air bottle and a hose.
 
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