The 3PH on my B7510HST does the same thing. I usually have the box blade hanging on the back for ballast when using the FEL. I have to check occasionally to make sure the BB is not hanging too low because my 3PH will slowly bleed down. Don't think there's any way to stop this using the tractor controls. Annoying, but not a fatal flaw. However, the LA302 FEL stays put without any bleed down, at least for a few days.