Asymmetry in the Clark Pump is when one stroke of the Clark Pump is not acting the same as the other. When you hear the Clark Pump shift, all the internal components move from one side to the other and it begins pumping water from the opposite side. As it is a fixed recovery system, symmetry between strokes is very important. When a Clark Pump has no internal leaks or internal wear the needle on the pressure gauge rises, spikes and falls at very regular intervals. When you see the pressure or flow reading act differently every shift (e.g. 90 psi <Clark Pump shift> 75 psi <Clark Pump shift> 90 psi <Clark Pump shift> 75 psi) there must be an internal leak in the Clark Pump where water is moving past a piston, valve, or seal when it should not be.

Below are the most common reasons for asymmetrical strokes in a Clark Pump:

  1. Annular Rings (Listen for a hissing noise on the low pressure stroke. This is an indication of a broken annular ring in the upper valve body.
  2. Annular Rings, check these again as they are a common culprit!
  3. The white seals on the reversing Valve Spool moving through the Annular Rings
  4. Piston rod lip seals (remove brass test port on 7 and 10% Clark Pumps). Slight drip is OK. Anything more means an issue with a rod lip seal.
  5. Cylinder walls. Small scratches will cause asymmetry and can be sanded or honed. However, if a cylinder has a score that can catch a fingernail, we suggest it be replaced.
  6. Piston Rod itself
  7. Check Valve assembly. The primary function of these is to hold pressure on one side of the Clark Pump. If the pressure on one Clark Pump stroke is 0 then these might be clogged, but they rarely lead to slightly reduced production. If the pressures are asymmetrical without a hissing noise, the most likely cause is a bad check valve in the lower center block.
  8. Pilot Valve Spool/Pilot Valve spool O-rings (To confirm this is the issue take out pilot valve spool, grease it and reinstall it, then see if the problem gets any better.)


Check out General Clark Pump Check-Up for basic tips on how to open up and evaluate a Clark Pump.

Refer to Clark Pump Kits & Reman Options for rebuild kits & factory overhaul options.