I have Dunlop K591 tires on the front and rear now and need a new rear tire. It looks like Dunlop discontinued the 130/80 size for the K591's. Is it safe to leave the K591 on the front and get a different rear tire? Is it safe to have a 130/80 on the front and a 130/70 on the rear?
Something tells me you don't have 130/80 on the front... stock size is 110/70/17 on the front and 130/70/17 on the rear.
Put a 140/70 on the rear and call it a day. I was planning on uppin to a 140 in the rear when I shopped for tires but if you're staying with a 130 on the rear, the choice is slim.
Building on what Wagoneer said, you should be fine so long as you don't mix bias ply with radial ply.
Radial rear with bias front can be ok but radial front with bias rear is not.
Are you sure that's a 130/80-17 you have on the front? Wouldn't fit inside a stock GS front fender, a 120/70 almost touches on the width and a 110/80 almost touches on the OD. Most of the K591 were belted tires as I recall and a belted front with a bias rear can be as bad as the radial front/bias rear combo.
With all the different brands, types, and sizes of tires I've run on the GSs a GT501 belted front with a Z2 radial rear was the only bad mix I ever had, felt like the bike was falling out from under me in spirited corners.
http://www.gs500.net/gallery/data/500/GS500tirelogs.jpg