The next time your pump is apart for cleaning, check all seals and o-rings for wear or damage. Over time they develop tiny cracks and need to be replaced. Further investigation depends on what style (age) your pump is…
For a Cast Valve with Captured Balls (1999 to present)
Make sure the balls move freely when cleaning.
A Cast Valve with Loose Balls (1993 - 1999)
Make sure the balls are in the proper place; one in the inlet port of the valve body and the other in the discharge port.
For Brazed Valve Bodies (prior to 1993)
Make sure a ball is installed beneath the ball-retaining pin, the retaining pin is in place and a ball is resting on the inner valve assembly under the cage.