Slow toilet refill: replace the fill valve or check the supply line first?

If the other bathroom fixtures have normal flow, test the toilet’s shutoff valve before replacing the fill valve. With the tank empty or nearly empty, close the valve gently, then reopen it fully. A partially stuck or obstructed valve can cause the variable refill you described.

Before disconnecting anything, shut off the supply and flush the toilet to relieve pressure. Then inspect the fill valve’s inlet screen or filter for sediment, if that model has one. Sediment there can restrict flow without the valve being mechanically failed.

Inspect the flexible supply line as well. A kink may be correctable, but corrosion, swelling, dampness, or a damaged fitting calls for replacement rather than cleaning. Don’t disassemble a corroded shutoff valve while it’s under pressure. If the valve won’t close reliably or begins leaking around the stem, replacing the valve is safer than forcing it.

Could a bucket-flow test help narrow this down before replacing parts? With the toilet shutoff closed and the tank flushed, disconnect the supply line at the fill valve and aim it into a bucket. Briefly opening the shutoff should show whether water reaches the bucket with a strong, steady flow. Weak or pulsing flow points more toward the shutoff valve or supply line; good flow makes a clogged fill-valve inlet screen more likely.

The valve should be closed again before reconnecting the line, and the bucket test should be brief to avoid a mess. Would you also check the hose washer and fittings for sediment while it’s disconnected?

For a beginner, start with the non-disassembly checks: make sure the shutoff is fully open, note whether other bathroom fixtures have normal flow, and inspect the accessible supply hose for kinks, swelling, corrosion, or dampness.

Disconnecting the supply line for a bucket test can disturb a brittle washer or marginal fitting and create a leak when it is reconnected. If those initial checks do not explain the slow refill, inspecting or cleaning the fill-valve inlet screen is usually less disruptive. The bucket test is still useful for separating a supply-side restriction from a fill-valve problem, but it makes more sense when the shutoff and line look sound and the valve closes reliably.

Screen access varies by fill-valve model. If the screen is behind the cap, follow that valve’s instructions before removing it, since the cap and seal need to be reinstalled correctly. A bucket test can still be useful when the shutoff and supply line look sound, but checking the screen first may avoid disconnecting an older supply-line fitting.

The model-specific screen instructions matter, especially because a seal or cap that isn’t seated correctly can leak once pressure is restored. After cleaning the screen or reconnecting the supply line, run one complete refill while watching the hose connections, fill-valve connection, and shutoff stem closely.

Even a small bead of water is a reason to stop using the toilet and close the shutoff valve, rather than assuming it will settle. A disturbed washer or brittle fitting may leak only under pressure, and replacing that washer or the supply line is preferable to leaving a slow leak concealed behind the toilet.

The pause near the normal tank level is worth separating from a genuinely slow refill. If water flows steadily after a flush but the valve slows or pauses only as the float approaches its set point, check whether the float is rubbing the tank wall, the adjustment is set too low, or the float mechanism is catching. That points more toward adjustment or a failing fill valve than the supply line.

A supply restriction usually affects the refill from the start, and the bucket test described above can help confirm that. I’d avoid adjusting the water level until checking that the float moves freely through its full travel, since changing the setting can mask a sticking mechanism.

I’d separate replacing the flexible supply line from replacing the shutoff valve. A damaged line may be a straightforward swap if the shutoff closes reliably and the connections are sound. Replacing the valve is a different job: the home’s water has to be isolated, and the new valve needs a secure connection to the pipe. If either step is uncertain, or the toilet valve won’t shut off reliably, that’s a good point to stop and get help rather than risk a leak.