The back brakes work better if the apportioning valve thinks there is a load on back of ute,so pull the spring up tight so the apportioning valve is full on, and you will notice the difference. The apportioning valve is there to stop the back brakes from locking up in slippery conditions,like ice or snow on roads, that doesnt happen often in oz and cars havent got them anyway, so I think you will be safe.
I have done this to my utes for several years now and have never had a lock up. If you dont do this this the front brakes do all the work and wear out heaps faster than they should.