One rear wheel lifting, is my max lift warning. I then either recenter the load, or just let the load slid along on the ground, and leave the wheel a inch or two in the air.
Looking at the parts break down, it only has one filter. I am thinking the "pile of grass clippings" fell in after he removed the filter. As others have stated, any grass that made it pass the filter should have been sucked into the engine when it was running.
Now personally, I adjust mine so much that check valves would be of little use to me. I look at them the way I look at the loader, if they are not quite where I want them I adjust them.
I do feel sorry for you, look on the bright side, in a year this will make for a great story. (y) (y)
I am inclined to agree with the other guys. But there is no harm in calling the dealer and checking, just to make sure. Plus that way if something does happen, they said you could use it.