On my Dads late model 4016 Vanguard,you cannot go forward without it slamming into gear like you popped the clutch.With the throttle up at cutting speed,no matter how slowly you try to engage the forward travel lever,all of a sudden it engages and the front wheels come off the ground.All linkages look good,figure it must be something in the main valve?Thanks.
The holding valve is built into the travel control valve. Carefully remove the holding valve and see if there is any crud in it, parts may fall out so be prepared. Clean the crud out. You may need a new O ring for the valve. The hydraulic system oil filter only removes the crud that stays in suspension in the oil return flow.
Also at the bottom of the travel lever where it passes thru the steering support bracket there is a spring and a nylon washer. This washer can get worn into an oval shape and cause some lost motion in this area causing an uneven application of forward travel to the control valve.
Plus what Hydriv said.
Mad Mackie in CT