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Militant Mk 1 Rear Air Brake Valve Leak.

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The air brake valve located on the N/S inside rear of the chassis was leaking through the exhaust port. I stripped the valve and cleaned the components with warm soapy water. The diaphragm appeared to be working correctly and not leaking when I blew into it through the threaded hole. I didn't replace the 0 rings as they seemed ok and, apart from some accumulated dust, the interior wasn't too grotty. I've now refitted this valve only to find that it's still leaking as before :cry:

 

I'm now wondering if there is still some problem with this valve or is it working correctly and reacting to a fault somewhere else within the system ? Any advice, thoughts and suggestions gratefully received. Cheers.

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Edited by 6 X 6

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I would check to make sure no air is getting through the foot treadle valve at rest with full air pressure.

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I would check to make sure no air is getting through the foot treadle valve at rest with full air pressure.

 

Thank you. It's difficult to maintain full air pressure due to the leaking rear valve but the foot valve appears to be working normally over 110psi.

Edited by 6 X 6

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Is it possible to remove the supply line to the valve and fit a blanking plug of some kind to the pipe to check there is no other issue?

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Is it possible to remove the supply line to the valve and fit a blanking plug of some kind to the pipe to check there is no other issue?

 

Yes, that might be worth trying. Thank you for the suggestion. I do have the same type of valve on another vehicle that has no air pressure faults so I've decided to fit this other valve to see if it also leaks. I'll report back on my findings.

 

Regarding other issues on the problem vehicle, one that I'm recommissioning after it has been standing for a number of years, there is a very interesting symptom in that there is only an air pressure supply to the windscreen wipers when the brake pedal is used. Something seriously amiss with the air system on this vehicle in it's present state !

 

Also, I may respectfully advise any Militant Mk 1 owners to check the condition of the 3/16" pipe that supplies air pressure to the gauge on the instrument panel in the cab. This pipe travels down beside the steering column, through the cab floor, and then along the outside of the chassis under the O/S front wheel arch in company with the armoured conduit carrying electrical cables to the rear..

 

I've seen a few of these steel pipes on Militants very badly corroded and brittle on the side of the pipe between it and the chassis. The side of the pipe on the outside looks ok but not so good on the back of the pipe that's not so easy to see. If this pipe is found to have deteriorated it's not difficult to replace the entire length of pipe from the gauge to it's source using 3/16" copper/nickel hydraulic brake pipe and fittings.

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I would only check other vehicle to see If valve has an standing air supply. If not your problem is with another valve on this vehicle do a pipeline chase to see where it is getting air from. It is many years since I chased air problems when I took parts off other vehicles I often then had a problem with that one as well usually a pipe fitting that did not want to reconnect.

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I would only check other vehicle to see If valve has an standing air supply. If not your problem is with another valve on this vehicle do a pipeline chase to see where it is getting air from. It is many years since I chased air problems when I took parts off other vehicles I often then had a problem with that one as well usually a pipe fitting that did not want to reconnect.

 

Ok. Thanks for that.

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Just an observation - you said it did not leak when you blew into it.

 

I can imagine all manner of diaphragm / housing leaks which might not show up at 6x6 lung pressure, but would at something approaching working pressure. Have you tried feeding it from a compressed air source?

 

What is the function of the valve - and does the valve accept full system pressure under normal conditions or just when brakes applied?

 

T

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If it's any use, this seems to be the same problem "ArtistsRifles" (Neil) is having. Two heads better than one?

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Just an observation - you said it did not leak when you blew into it.

 

I can imagine all manner of diaphragm / housing leaks which might not show up at 6x6 lung pressure, but would at something approaching working pressure. Have you tried feeding it from a compressed air source?

 

What is the function of the valve - and does the valve accept full system pressure under normal conditions or just when brakes applied?

 

T

 

Thank you, NOS.

 

The user handbook describes this component as a relay valve.

 

I blew into the diaphragm while it was separated from the main housing (please see 'photo) to check for any obvious leaks or tears and to see if the very small air way (at the bottom of 'photo) was clear. I doubt if this diaphragm could with stand 120psi when not supported in it's housing.

 

I'm not sure what part this relay valve plays in the system and would be wary of connecting it to any source of air pressure other than that provided by the vehicle it is intended for. I'm hoping to find time this weekend to fit another valve of the same type to see what, if anything, this might reveal.

Relay Valve A.jpg

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If it's any use, this seems to be the same problem "ArtistsRifles" (Neil) is having. Two heads better than one?

 

Yes, I certainly agree that two, or more, heads are better than one especially if the one head is mine. Coincidentally, on the very same day last week that "AristsRifles" was removing the foot control valve on his Mk 1 I was doing the same job on my Militant:shocked:.

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Thanks for the description.

 

Similar issues with a Constructor, where a relay (shuttle) valve, fitted between brake application valve and brake cylinders, and with a line to the palm coupling would sometimes let air through to service brake lines, often after running for 10 minutes or so.

 

What happens is that air pressure gradually builds up in the line to the palm coupling - when it equalises, the shuttle valve thinks that it has received a signal to apply brakes and opens up the line to service brakes, effectively bypassing the foot valve.

 

The cause is usually attributable to blocking up of a tiny bleed hole in the face of the palm coupling blanking plate (paint often being the culprit). It allows any slight leak of air into the service line (from whatever valve) to disperse instead of gradually building up.

 

I wonder if your Militant relay valve is doing a similar job, in that it responds to signal pressure from a brake line coupling from a remote vehicle and feeds / directs the vehicle's own air supply to brake lines / through line to service coupling?

 

Suggest you try removing any blanking plates from the couplings and see if that cures it. If not I'm officially flummoxed!

 

I guess this bleed hole prevents minor valve leaks becoming a problem, but maybe you have a more serious leak from another valve somewhere in the service lines? In which case it has to be a valve or device which has both supply and service line connections.

Edited by N.O.S.

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Been watching this thread with a good deal of interest, as I have 2 Militants, neither of which is fitted with this valve, and it doesn't get mentioned in the parts books or handbooks I have for them.

 

If I am understanding it right (no Guarantee on that) there should be no pressure at all here unless the footbrake is being operated. This would suggest, either the footbrake valve is letting air pass, not enough to apply the brakes but enough to open the valve. Or air is being back fed through the emergency line and leaking straight out of the exhaust port.

 

As for the 3/16" pipe to the dashboard gauge, I've had to replace both of mine. The Timber Tractor was blocked solid, and the one on the Tanker was only held together by many layers of green paint. Easy to replace, though, with standard car brake piping

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Welcome back 36, where have you been?

 

From your diagram the valve is in the system as a trailer relay valve, 5 is the constant pressure from what we would now call the red airline, 4 is the feed to the yellow airline and 2 is the trigger pressure from the brake pedal.

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Welcome back 36, where have you been?

 

From your diagram the valve is in the system as a trailer relay valve, 5 is the constant pressure from what we would now call the red airline, 4 is the feed to the yellow airline and 2 is the trigger pressure from the brake pedal.

 

So is there an equivalent to the Constructor's 'shuttle valve' somewhere in there between the palm coupling and this relay valve, Mike, which might be the culprit?

 

Tony

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Thank you for taking the time to post so many helpful suggestions. This is the HMVF at it's very best :-D I'll reply in detail in a few days time as I'm presently preoccupied preparing a vehicle for an important local midweek show .......might see you there.

https://www.steamheritage.co.uk/steam_rallies_and_events/listing/noggin_nosh_natter

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So is there an equivalent to the Constructor's 'shuttle valve' somewhere in there between the palm coupling and this relay valve, Mike, which might be the culprit?

 

Tony

 

I am looking at the brake layout for the 30 ton Constructor tractor but cannot see any device between the rear 4 palm couplings and the relay valve.

 

There are a pair of change over valves associated with the front palm couplings, look to be mounted just behind the foot brake reaction valve inboard of the steering servo.

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I am looking at the brake layout for the 30 ton Constructor tractor but cannot see any device between the rear 4 palm couplings and the relay valve.

 

There are a pair of change over valves associated with the front palm couplings, look to be mounted just behind the foot brake reaction valve inboard of the steering servo.

 

Sorry - I meant were you aware of any similar valve on the Militant system!

 

Yes, it was one of those front changeover (shuttle) valves causing the pressure build-up on the line feeding the Constructor rear brake chambers - I can't imagine the Militant system is that different.

 

We'll have to wait for 6x6 to put his toys away after the show before any further online diagnostics.

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A Militant is Air only brakes, If I remember correctly. A Constructor is an Air plus Spring parking set up. Foot valve on the Militant is very basic, the Constructor is next generation control with pressure proportioning and load sensing. I could not wait to see the back of palm couplings. I should have asked what is the valve for. If it is a trailer supply it is either getting a signal to feed air or it does have a fault and is just leaking. If it is the end of the line and you are not going to run a trailer, there should be no problems blanking it off that will let the system fill properly and the relief valve will cut in at max pressure. Then at least you will have brakes on the truck, and can work on a fix for the valve at your leasure.

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Sorry. I had the pleasure to drive a Gardiner engined 8x4 Constructor with a David Brown range/splitter box in the 1990s. My mind did not go back past that.

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I just came back from the noggin, nosh and nattter, I guess you didn't make it?

Phil

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I just came back from the noggin, nosh and nattter, I guess you didn't make it?

Phil

 

Hi Phil, I was there with a Series 2a ambulance parked next to those Champs. I had been hoping to take the Militant but there was no way it would be on the Public Highway with any air leaks or other mechanical issues. I'm always amazed that any Matador has survived intact when so many were to converted to recovery or timber work. I look forward meeting you at some future event and hearing the story behind your magnificent AEC. Cheers, Tom:-)

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Success At Last !

 

It occurred to me that as this leaking relay valve relates to the rear trailer brake couplings it must be controlled by the trailer brake lever mounted on the steering column. Also, the fact that the windscreen wipers weren't operating correctly (see post five on this thread) and they sourced their air pressure supply from a pipe connected directly to trailer brake valve the cause of the leaking rear valve may very well be here.

 

Having built up the air pressure to about 100psi I worked the trailer brake lever on, and off, for around a dozen times after which built up the air again and NO MORE LEAKING REAR RELAY VALVE. It was as simple as that. The system now quickly builds to 120psi and holds this pressure for a considerable length of time.....certainly long enough to satisfy both me and VOSA.

 

Maybe, if not ready doing so, Militant and owners of vehicles with a similar system might like to give both the trailer brake valve, and it's related rear relay valve, a quick work out by applying the trailer brake lever on and off as part of routine maintenance.

Public Service Warning:apply the hand brake before attempting this procedure. All work should be undertaken by a qualified mechanic.:-D

 

Thanks for all the advice and suggestions.....very much appreciated !

 

 

 

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Edited by 6 X 6

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