Drafting water from a creek or pond is standard procedure, but when your truck refuses to hold prime or loses suction immediately after shut-off, the culprit is often the suction line integrity or the foot valve.
Understanding the Vacuum Physics
For a water truck pump to work, it must create a vacuum of approximately 0.085 MPa or higher depending on the lift required. If the foot valve (a check valve at the bottom of your hose) leaks, water rushes back into the pond, breaking this vacuum.
Symptoms and Diagnostics
Loss of Prime: If you have to manually fill the pump casing with water every single time you start the truck, your foot valve is leaking.
Cavitation Noise: If the pump sounds like it is pumping gravel (“marbles in a can”), you likely have a blockage. Standard specifications require the pump inlet to have a filter mesh. If this mesh is torn, debris like rocks can lodge in the foot valve or impeller, restricting flow.
Operational Solutions
Valve Logic Check: Modern trucks use complex 4-way 3-position ball valves (often labeled A, B, C, D positions). To draft water (suck from a pond), the valve must be strictly set to the “Suction” position (often Position B in TLS schematics), connecting the suction hose to the pump inlet while closing the tank outlet. If the valves are partially open or aligned to Position A (Closed), the pump will cavitate and fail to prime.
The “Wet” Test: If you suspect a foot valve failure but need to finish the shift, try closing the Manual Ball Valve at the pump inlet immediately after shutting off the PTO. This traps water in the pump casing, acting as a temporary manual check valve.
Maintenance Note:
Inspect the pump’s mechanical seal if you see water dripping from the shaft area. A damaged water seal allows air to enter during the suction phase, killing the vacuum. This often happens if the pump is run “dry” for more than 1-2 minutes.
