water is leaking from the bottom of the pump, near the shaft, when the machine is running.

Diagnosis

The leak you are seeing at the bottom of the pump housing, specifically near the drive shaft, is a definitive sign that the primary axial water seal has failed. In the DeWalt DWPW2400, the pump uses a series of rubber O-rings and high-pressure seals to keep water isolated from the internal lubricant and the motor shaft. When these seals degrade—often due to "dry running" (running the motor without water) or the ingress of grit and sand—water bypasses the seal and collects in the pump sump. Because the pump housing is designed with a weep hole or a drainage gap at the lowest point, the water doesn't flood the motor immediately; instead, it drips steadily from the bottom of the unit during operation. You can confirm this by running the machine for two minutes and checking if the fluid is clear water or a milky-white emulsion. If it is milky, the water has already breached the seal and mixed with the pump's internal gear oil, which means the seal is completely gone and the lubrication is compromised.

How to Fix It

For the DWPW2400, which typically utilizes the AAA-style pump assembly, replacing individual O-rings is rarely successful because the seal seat often becomes pitted. The most reliable fix is replacing the entire seal plate assembly.

1. Disconnect and Drain: Unplug the unit from the power source. Use a basin to catch the remaining water and oil. Remove the pump head bolts (usually four 10mm or 12mm bolts) using a socket wrench.
2. Disassemble the Head: Gently pull the pump head away from the manifold. If it is stuck, tap it lightly with a rubber mallet. Be careful not to warp the aluminum casting.
3. Remove the Seal Plate: Locate the seal plate (the plastic/composite disc between the head and the motor housing). Remove the retaining screws and slide the plate out. Inspect the shaft for any deep grooves or burrs.
4. Install New Seal Plate: Slide a new OEM seal plate assembly into position. Ensure it sits flush against the housing. If there is any debris on the mating surface, clean it with a lint-free cloth and a small amount of isopropyl alcohol.
5. Lubrication: Before reassembling, apply a thin layer of high-pressure waterproof grease to the new seals to prevent "dry start" friction, which can tear a new seal instantly.
6. Reassemble and Torque: Replace the pump head. Tighten the bolts in a star pattern to ensure even pressure across the gasket. Torque them firmly, but do not over-tighten, as the aluminum threads in the DWPW2400 housing can strip easily.
7. Prime the System: Connect your garden hose and run water through the machine for 60 seconds *without* turning the power on to purge air from the pump head.

If That Didn't Work

• Inspect the Crankshaft for Scoring: If the leak persists, remove the seal plate and inspect the metal shaft. If you see circular grooves or "scoring" etched into the metal, the new rubber seal cannot form a watertight bond. In this case, you must either polish the shaft with 1000-grit sandpaper or replace the entire pump assembly, as the shaft is a non-serviceable part.
• Check the Manifold Gasket: A leak appearing at the bottom can sometimes be a "traveling leak" where water escapes from the manifold gasket at the top and trickles down the side of the housing. Replace the perimeter O-ring on the pump head to rule this out.
• Verify Inlet Valve Integrity: Check the water inlet screen for debris. If the inlet valve is partially blocked, it can cause cavitation (vacuum bubbles), which creates extreme heat and rapidly destroys new seals within minutes of operation.
• Replace the Entire Pump Unit: If the housing itself has a hairline crack (common if the unit was stored in freezing temperatures with water inside), no amount of seal replacement will stop the leak. You will need to swap the entire pump assembly.

Heads Up

• Never Run Dry: Never start the motor without water flowing through the pump. The seals rely on water for cooling; running the DWPW2400 dry for even 30 seconds can melt the rubber seals.
• Winterization: Always use a pump guard or blow out the pump with compressed air before winter. Frozen water expands and cracks the seal plate, leading to the exact leak you are experiencing.
• Water Filtration: Install an inline sediment filter on your garden hose. Small grains of sand act like sandpaper on the internal seals, shredding them over time.
• Oil Check: If you saw milky water leaking, you must change the pump oil immediately. Water-contaminated oil will cause the internal pistons to seize.