Have you adjusted the back support all the way back? I found the stock location is somewhere in the middle of the range. When I adjusted mine I moved it back and then banged on the bumper to move it back just a little bit more to lock it in the farthest back location.
Yes, the bum bumper can be removed. Open the seat. Look at the 2 rods that can be seen for the support. There are 2 set screws in the left side rod (just looked on mine). Remove those set screws and the bumper should slide out while sliding and holding the lock lever to the left.
