Have you tried this one?
http://www.kleen-ritecorp.com/p-1549-fluid-controls-ball-bearing-swivel-38fxm-brass-2000psi.aspx
I added this one to my HP hoses and it made a huge difference. No more binding hoses. There have been many threads started about which swivel is the best, and I think these have been the most popular.
Do you use a 45° elbow?
On the inlet end of the fb handle I have the swivel then a street elbow then the hose into the elbow.
The Spraying Systems swivel I linked turns very, very freely. It's so free I was concerned at first that they wouldn't last, but they hold up great. I was using the economy ones from Kleen-Rite before, and they'd only last about six months, and they never were as smooth or free.
It does help if your brushes hang horizontal and you have issues with the hose kinking or the bend restrictor breaking. I prefer the vertical hanger with the bracket way up on the wall so little kids can't get the brush down to slop it in mud. They seem to really like doing that.I will give one a try. I also don't use elbows going into the FB hose. Don't know if it would make a difference or not.