There is a small diaphram pump located on the bottom. It is driven directly off the motor shaft by means of an offset pin. The top of the pump is some kind of soft plastic material that dries out and cracks. Attempts to repair with epoxy have failed. While the design is good, the choice of materials was poor and I'm sure that every unit that's over ten years old has failed or been in storage and will fail if plugged in. The previous post suggested an external pump and that's exactly what I did. The whole unit is easy to disassemble once the top cover is removed. First, remove the stone holding the nut with a pair of pliers and a flat blade screwdriver. This is a left-handed screw. Now, exposed are 3-screws around the shaft and four around the periphery. Once those are removed, there are 5-long screws in the black case. Cut the label with a sharp knife as it sits across the two body halves. The remove the 5-long screws and pry the case apart. This guy is well made inside. To get to the pump, you will have to remove one screw at the base. It's between the two plastic hoses. To get to the screw, you will have to remove the 4-screws and plates holding the motor to the case. This is easy to do.
Now lift the motor about 1/2" and you can now remove the pump screw and entire pump assembly.
You will probably see a tear in the diaphram. If you can repair it, go ahead and try. I gave up after the using a two-part expoxy.
So then I pulled the hoses, reassembled the whole thing and connected up a small aquarium pump. As it is well made, the reassembly goes together without any problems.
Good luck. One might also try calling the 800-number shown on wenproducts.com during the work week.
Tuck
