Since you have brass bushing in your wheels, its possible that the wheels are twisting sideways when rolling on the grass. With the plane inverted on the bench, apply force to the wheels from the front. If the wheels are twisting.. loosen your struts and turn till the wheels are straight and then tighten. I had this problem with my Prolinks that ARE trailing links. The bushings started to give and when taxing they acted like the brakes where on. It took a lot more effort to get in the air.