the clean water tank on my dock empties but the dirty tank doesn't fill, robot just skips the wash cycle.

Diagnosis

The issue where the clean water tank empties but the dirty water tank remains empty is a classic sign of a "leakage or blockage loop" within the Roborock S8 MaxV Ultra dock's internal plumbing. When the dock initiates the mop washing sequence, the pump activates and pushes water from the clean tank into the washing tray. If the dirty water tank isn't filling, the water is either failing to reach the tray entirely (meaning it's leaking internally into the dock base) or, more commonly, it is pooling in the washing tray and failing to be vacuumed back up into the waste tank. This usually happens due to a failure in the suction seal of the dirty water pump or a blockage in the drainage grate at the bottom of the dock. If the robot skips the wash cycle, it is because the dock's sensors detect a "water recovery error"—the system knows it dispensed water but didn't see the corresponding rise in the dirty tank level, triggering a safety shutdown to prevent the dock from overflowing and damaging your flooring. You can confirm this by checking the washing tray after a failed cycle; if there is a shallow pool of water left behind, the issue is the recovery suction, not the delivery pump.

How to Fix It

To resolve this, you must clear the drainage path and prime the recovery pump. Follow these exact steps:

1. Power down the dock and remove both the clean and dirty water tanks.
2. Inspect the rubber gasket on the bottom of the dirty water tank. If there is a pinch or a piece of debris (like a clump of hair or dust) on the seal, the pump cannot create the vacuum necessary to pull water up. Clean the seal with a damp cloth.
3. Locate the drainage grate at the bottom of the dock's washing tray. Use a toothpick or a small brush to clear any slime or debris trapped in the small holes of the grate.
4. Fill the clean water tank with 500ml of distilled water mixed with 50ml of white vinegar (approximately a 10:1 ratio) to break down mineral scale.
5. Using a 20ml syringe, draw up the vinegar solution and manually inject it directly into the clean water intake port inside the dock to force a prime through the lines.
6. Reinstall both tanks and ensure the dirty water tank is pushed down firmly until it clicks; a loose fit will prevent the vacuum seal from forming.
7. Open the Roborock App, go to Settings > Dock Settings, and trigger a "Manual Mop Wash." Observe the dirty water tank to see if the water level rises during the recovery phase.

If That Didn't Work

• Check the Float Switch: Inside the dirty water tank, there is a small plastic float that detects the water level. If this float is stuck in the "up" position due to grime, the dock will think the tank is full and refuse to suction water. Gently shake the tank or use a pipe cleaner to ensure the float moves freely.
• Deep Clean the Recovery Pipe: If you see water in the tray but the tank stays empty, the internal recovery hose may have an air leak. Use a wet-vac or a high-powered handheld vacuum to suction through the dirty water intake port from the outside to clear any stubborn clogs of "mop sludge."
• Reset the Water Sensors: Power cycle the entire unit by unplugging it from the wall for 60 seconds. This resets the pressure sensors in the pump assembly which may have been tripped by an air bubble (airlock) in the system.
• Inspect the Base Seal: Check the bottom of the dock for any signs of water leakage. If water is pooling under the dock, the internal tubing has likely disconnected or cracked, and the unit will require a professional replacement of the internal pump assembly.

Heads Up

• Never use foaming detergents or thick cleaning agents in the clean water tank; these create air bubbles (cavitation) that trick the sensors into thinking the tank is empty and can permanently damage the pump.
• Empty the dirty water tank immediately after every few cycles. Leaving stagnant, dirty water in the tank creates a biofilm that clogs the recovery suction pipe over time.
• Use distilled water exclusively if you live in a hard-water area. Calcium buildup in the S8 MaxV Ultra's narrow internal tubing is the primary cause of pump failure.
• Periodically remove the mop washing tray (if applicable to your specific regional dock version) and scrub it with a brush to prevent sediment from blocking the drainage holes.