I've been using compression springs for a while to try and keep the handbrake working but during the last rear end overhaul to replace bearings etc I took another look at the mechanism. I put a lot of effort into minimising the disc runout, setting up the centralising strips and setting the pad to disc gap with a few other things ... and it works! The handbrake has not needed adjusting this year.
Previously I did have compression springs pushing the two halves apart (and I've kept them handy just in case!). I bought some new ones which are still in the packet and they say Eliza Tinsley (the brand) 2" long by 7/16" wide (just fits over the rod) by 0.054" which I think is the wire thickness. The 2" bit is too long and I cut them down but I don't have the old ones handy to say by how much. They came from my local agricultural supplies shop so they're probably a Massey Ferguson part
Stuart