When we load our tankers we have to drop the yellow airline to get get a more accurate reading on our weigh gauges
Does anyone know if it would be legal to fit a tap at on end of airline to turn off when loading instead of dropping airline.
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The short answer is no, as taps are illegal.
Being old, I can well remember palm couplings and taps and it was only too easy to
forget to turn the taps on after coupling up.
With the parking brake set, some tractor units pressurise the yellow service brake line and some donāt.
For example, Mercedes do and Volvo donāt.
As far as I am aware, this means that the trailer service brakes are either on or off.
In any case, a tap would make no difference, if pressurised, the brakes would still be on and if not pressurised, the brakes would still be off.
Admittedly taps were a safety liability.To be fair palm couplings with the similar integral valves as valved C couplings were/are the best and easiest to use compromise.Bearing in mind that non valved C couplings were also used with taps.
What does the yellow line do? In multi trailer operation, taps are a necessity, Iāve never driven off with the taps closed.
In Europe, the red line keeps the spring brakes off and the yellow line is the service brake air supply that puts the brakes back on again when the brake pedal is pressed.
I have once driven off with the taps closed, I was in too much of a hurry and I realised the first time I braked. It was in the 1970ās and I was younger and dafter then.
In those days, British trailers had three air lines, the third being the blue emergency line, which was always a suzie and not a palm coupling.
The parking brake was a ratchet and cable that applied the brakes on the front axle.
If there was no air in the trailer and no one had applied the parking brake, the trailer was free to roll away.
Nowadays, european trailers have two valves on the trailer, operated by buttons. One applies and releases the parking brake and is red and the other is a shunt valve, that lets the trailer brakes off for yard shunting purposes, without the need to connect any air lines. That one is dark blue, or black.
Both rely on there being air pressure in the trailer and if there is none, then the spring brakes will automatically come on and the trailer will not move.
On the most modern trailers, if the trailer air pressure goes down or the red air line is removed, the parking brake is automatically applied and the trailer will not move until the red button has been pressed in again.
This is a very good idea and long overdue. Drivers have been embarrassed, injured and even killed by their vehicle rolling away after the red line was connected, because they had forgotten to apply the tractor unit parking brake.
There is not much else that the manufacturers can do to ensure safe operation, the only way that a driver can do things wrong and be able to drive away, is to fail to connect the yellow line, which I would imagine happens very rarely, if at all.
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Thanks for the explanation ncooper00, thats actually the same as we use, except we use red for supply and blue for signal. The yard release button is optional.
Did you ever experience the joys of vacuum, or later air over hydraulic?
Iām not old enough for vacuum brakes, which a friend of mine tells me were in the 50ās
and before and the only air over hydraulic I ever drove was a Leyland Terrier, 7.5 tonner.
I remember that the brakes were either on or off, so it was very difficult to stop gracefully.
I was reminded of the āCaution, air brakesā sign that you could see on the back of some lorries
when I was a child.
Apparently, it was there because for the first time, a lorry could stop quite quickly. 
With trailers lasting, almost forever, we had air and vacuum over into the '70s, however most had been converted or replaced.
When I started driving I did drive some vacuum braked trucks. As I was inexperienced I could not compare them much with air. When I got air brakes it must have been better but driving steady with bagged coal on local runs did not require Jumbo Jet standard brakes anyway.
That would have been late 50s and early 60s trucks. They were not new tho` I am not (quite) that old.
There was a saying in the bus world that AEC vacuum brakes were better than Leyland air brakes.
The biggest issue with vacuum brakes was always the size of the reservoir and ability to maintain enough nothing. 
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During the bad winter of 62/63 I was driving a Thames 4D (Ford) on bagged coal deliveries, as the unleaden weight was less than 3 tons a car licence was OK (I was not yet 21) and that had vacuum over hydraulic brakes which were rubbish carrying 5 tons. One morning I moved another drivers lorry and the brakes seemed better than mine so I arranged to go to the local Ford commercial dealer for a check. Long story short I picked the lorry up and drove bak to load, going a bit staedy as local road was frozen, after loading was driving on a cleared road and applied brakes at normal effort, we stopped better than many air braked Iāve driven since and my mate hit his head on the windscreen. So some vac/hydraulic systems were OK. Went on to drive many air over hydraulic systems like TK Bedfords but no probs there.
Palm couplings had suzies too. It was just the connector that was different. The advantage of palm couplings was that they were a lot easier to attach but the disadvantage was they came off more easily. A lot of UK drivers were put off palms because on dock traction you ended up with adaptors to convert bayonette fittings to palms, which extended the workings and gave suzies an excuse to snap if brittle.
Iām sure that palm couplings arenāt a āwasā they are still the default choice outside UK ?.
We made the wholesale change to palms after the UPS takeover then later changed back to the usual UK C bayonet type.
The main problem I found with palm couplings was just the tendency to kink air lines when twisting them to couple and uncouple.The lines could be fitted with metal spring cover guards which worked fine and I much preferred palm couplings.
Sorry, I meant a male C type coupling on the blue line (suzie).
If I remember correctly, it was a larger diameter than the ācontinentalā
C type coupling, presumably so that it could not be wrongly connected.
I do remember the adaptors and also the fact that many UK tractor units,
like the diabolical Leyland that I was driving, were too short for the long pinned āTIRā trailers.
The adaptors just made things worse in the tiny amount of space between the cab and the trailer headboard.