The check valve is simply a piece of plastic slightly larger than the
intake hole. It is held in place by a piece of tape that also acts as
the hinge. It's located just below the intake flange at the top of the
suction box. It's quite easy to dislodge. Perhaps that happened to you.

To do a 5-minute repair, you'll need to remove the bellows so you can
get at the screws that hold the box in. Now unscrew them so you can
take out the suction box. Now remove the flange so you can fix it.
Reverse the process and put it all back together.

Jeff Davis