There should be a check valve ( or two ) and a filter in the engine compartment, or all the fluid will run back between uses,

I think the check valve is red and black, and contains a small spring and a thin rubber flap.

I have seen the flap tear or turn to gunk if the cleaner eats it up.

The valve just snaps apart ( don't lose the spring! )

A small disc cut from a dish washing glove collar serves well for years - your wife won't even miss this small piece.

Have you read the FAQ?