What was wrong with the check valve?

Just pop it apart - don't lose the spring - all there is in there is a small spring and a rubber flap.

The flap is either torn, or it has turned to sticky goo.

Make a new flap from a tiny piece cut from a dish washing glove cuff - your wife won't even notice it. I have had this simple repair last for years.

With no check valve, the fluid may siphon back to the tank, and when you go to wash the rear window the wiper will run dry on the dirt for some time.

Some times trying to save money is just cheap.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '72 Yamaha Rd400, '68 Honda 350-4, '12 XC70, the first 5 are mine, heh, heh, 525,000 miles put on 10 bricks James A Sousa