Have a look at this Chris, something I was unaware of until 6x6 pointed it out, I would think those valves would have much the same effect!
I'm pretty sure this was the problem with Jule's Militant brakes as well, can't find the post though.
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Obstructed breather vents on the front palm couplings may not be the cause of your brakes locking on but I hope it is because it's a problem that's so easy to remedy. These vents are usually a small hole in the centre of the depression on the dummy pressing which locates on the coupling when it's not in use. These holes can become blocked by paint. The other type I'm aware of is a primitive one way valve in the form of a flexible washer at the same location. This sort can also become gummed up with paint or old age.
Being unable to release air pressure through the front couplings causes the vehicle you are driving 'think' it's being towed by another lorry which has, via air lines, just applied it's brakes. Typically, this fault occurs at traffic lights on busy main roads and just about any where else where it will cause the maximum chaos.
And from NOS
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I think you might have just solved an annoying problem with a certain RAF Constructor - thanks 6x6 !!