Mini Excavator Removing Big Rocks

It seems to me like when I keep the thumb pedal pressed (opening the relief valve) the thumb pressure on the rock is maxed out, but when I first let off the pedal, the thumb backs off just slightly. It's strong enough to still clamp the rock, but not enough to lift it unless I have a really good angle on it. When I look at the following video (at 4 seconds in), it seems like the thumb clamps on the rock and never bac

Discuss this in the forum topic Kubota U17 Thumb Install.