Possibly Prochem, since it was some of the Prochem lads who set up Blueline so Iv'e heard and they are very similar in design so may even share the part.
But, first take it apart. It might just be clogged up with crap, it's probably a simple spring and wax valve affair, could just be stuck.
It might have a make & part no on too.
Adrian, your a member of truck mounters, why didn't you ask there ?.