my first semi build (dump)

[steamer]

If your mounting point on the slider block and the dump box are on the same plane or level you are right it would never lift. You'll have to pick a spot on the slider that is as low as you can mount the push rod. The same goes for the dump box mount. Try to get mount as close as posible to the floor of the box, maybe even build up the box frame. That way it won't try to push strait out, but will have some angle for leverage to start the lift.