The LSPV spring diverts more hydraulic fluid to the rear brakes depending on load/weight distribution.
I use a breakers yard for most of my bits in England called "5 star Jap parts". I phoned them and they want £20 for a used unit. They advertise on the net too and can send parts to you as well. Hope this helps..
P.s what makes you think yours is faulty?
I have one on my cut previa trailer that I am going to remove. see it here
http://www.youtube.com/watch?v=mI7M1ixYaf4