HI I drive a MAN TGA410 on a 52 plate after applying the parking brake the air in the trailer leakes out of the trailers emergency relay valve,I had a word with the fitter about this and he had the valve changed, the next day it was still leaking, had another word and he said it’s just one of those things.If i get a pull from the ministry he is not going to be happy .Had a delivery to a place the other day and the boss asked if i minded thier fitter had a word with me about the truck ,He came out and said they had the same problem,He also changed the ERV and the same thing happened,we noticed that the service suzie had presure in it when the parking brake was applied is this normal for this truck? Has any one else had this problem?If so what is the cure?
Ooops, thought you were on about the yellow line. 
Dunno about the valves.
Is it the yellow or red line ?
Service, yes, your on about the yellow line.
I’ve noticed the 52 MAN has pressurised yellows, nothing but trouble.
DAF’s fit them too, can’t work out why, though.
The yellows stay pressurised with handbrake and footbrake on. Whilst in cab and engine off, put in gear, let off handbrake (no foot on brake pedal) to see if leak stops.
Sorry, only cure, take yellow off, when parked. 
DAFMAD:
Sorry, only cure, take yellow off, when parked.
Or apply trailer brake and release parking brake. 
As to why they configure them that way? I’m not sure but someone, can’t remember whom, recounted a tale on here a couple of years ago of when they were tipping with an artic.
As the load began to move off, the weight transfer caused the drive axle to lift clear of the ground, and because he hadn’t got his foot on the brake pedal, the whole vehicle was pushed forward into a parked car. 
Obviously, a pressurised Service line would not allow that to happen. 
Some yellow airlines are pressurised when the park brake is on, it depends what make of truck, what year of manufacture etc
Technical bit coming up!
The line is pressurised because the trailer, rather than parking on spring brakes, i.e no air, they park on the footbrake, so when you apply the park brake in the cab a valve sends a supply of air down the yellow line, to park the trailer, the same as if you had your footbrake on full, if the air drops or you disconnect the trailer the red airline pressure is released and the spring brakes come on
I personally would not disconnect the yellow line when parked for a number of reasons
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what if you forget to reconnect it before driving off in the morning.
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by discconecting the yellow line only you are now parked using the unit brakes ONLY, not good fully loaded on a slope!
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either leave tham both on, and loose your air or remove both airlines.
which valve is leaking? I do not fully understand which valve you are on about, oh and “its just one of those things” is not acceptable a leak is a leak! and it should not be there. 
If its the valve i think you are on about, if the park and service circuits go through it, it could be a brake chamber diaphragm leaking, as when the service brake is on, if air leaks through to the park or visa versa then it will leak through said valve, does it only leak when parked or does it do it with the park brake off?
It is the yellow line and the valve on the trailer (the emergency relay valve )only leaks when the parking brake is applied the air comes out at quite a rate until it gets to about 6 bar or just when there is not enough air to keep the spring brakes off some times if you stamp your foot on the foot brake it will stop the air from leaking and keep the pressure up.