http://www.militaerbestaende-schult.de/KFZ-Ersatzteile/Sonstiges/Bremskraftregler-WABCO.html?listtype=search&searchparam=bremskraftregler
The part is also used on some DAF vehicles but my local branch has been less than helpful.
A chap in Germany rebuilds them, not for the faint-hearted, for about 150 euros.
A breaker is the only other way, but ensure you keep your lever and spring. There was one body, but ten springs and two levers across the whole range.