COMPATIBILITY FIRST · NO INTRO ADS
Replace a fill valve
Fluidmaster 400A bottom-entry fill valve in a compatible conventional two-piece gravity toilet. The illustrated supply is a flexible braided connector with its own captive seal.
- Verify the toilet maker permits a 400A; Fluidmaster recommends this model particularly for older 3.5–7 gallons-per-flush gravity toilets.
- The installed C.L. mark must be at least 1 inch above the overflow opening, with clearance under the lid and around the float.
- Use the new complete valve and shank together; do not mix a new body with an old shank or move the locking ring.
- Replace a worn supply connector or one over five years old, using a connector compatible with both fittings.
- The existing overflow top must also be at least 1 inch below the bottom of the handle opening. Stop if the required height clearances cannot all be met.
The animation illustrates the named mechanism. Use your exact fixture and replacement-part instructions if their design or sequence differs.
01 / DIAGNOSE BEFORE BUYING
Confirm the repair fits the problem.
- Water overflowing into the overflow pipe suggests a fill-level, float or fill-valve problem.
- Brief refills while nobody uses the toilet usually indicate tank water escaping through the flapper or flush valve; do not assume the fill valve is faulty.
- Slow or absent refill may be debris or a restricted shutoff/supply. A model-specific cap-cleaning procedure may repair a sound 400A.
This procedure does not cover
- Side-entry fill valves
- Pressure-assisted, tankless, concealed, electronic or specialty tipping-bucket systems
- One-piece or low-profile toilets not confirmed compatible by the toilet and replacement manufacturer
- Rigid supply connections needing different sealing parts or skills
02 / COMPATIBILITY & PREPARATION
Have the matching parts ready.
- Replacement 400A with new shank washer, locknut, refill hose and clip
- Bucket, sponge and towels
- Adjustable pliers for removing old nuts
- Scissors and ruler
These checks are for your use. They do not certify compatibility and are not saved.
03 / STRAIGHT TO THE REPAIR
See how the parts move.
Read the narration transcript
Shut off the water, flush, and make sure the tank stops refilling. If the stop valve leaks or will not close, stop here. Set the lid safely aside and sponge the tank dry. This example uses a compatible Fluidmaster 400A bottom-entry valve. Put a bucket below the tank. Disconnect the supply, remove the old locknut, and lift out the valve. Fit the new shank washer flat side against the valve. Adjust its height without moving the lock ring: the C.L. mark must be at least one inch above the overflow opening, and the lid must clear. Seat the valve and hand-tighten its locknut. Keep the float clear. Clip the refill hose above the overflow opening; never push it down the pipe. Reconnect the supply by hand using its correct seal. Turn on the water and check for leaks. Adjust to the toilet's water line, safely below the overflow. Flush and check again. The valve must shut off and every joint stay dry. If it leaks, turn the water off and correct the seal rather than force the nut tighter. If it refills on its own, check the flapper for escaping water.
04 / THE PROCEDURE
Work gently. Check as you go.
- Isolate and empty Protect the lid. Note the toilet's proper water-level mark. Close the stop valve gently, flush, and verify the tank does not refill. Sponge out the remaining water. Stop if the supply cannot be isolated.
- Remove the old valve Put a bucket below the connection. Disconnect the supply at the tank, then remove the fill-valve locknut and lift out the valve, old shank washer and refill tube.
- Set the height Fit the new shank washer flat side against the valve. Twist the lower shank to adjust height without moving the lock ring. Set the C.L. mark at least 1 inch above the overflow opening; on this 400A the top of the cap is 3 inches above the overflow opening. Check lid clearance.
- Secure and route Seat the valve with the rubber washer inside the tank. Hold it down while hand-tightening the locknut underneath. Leave the float clear of all moving parts and tank walls. Clip the refill hose above the overflow opening so water enters it; never push the hose down the pipe.
- Reconnect and fill Reconnect the flexible supply with its correct captive seal and hand-tighten the tank coupling. Turn the water on and check every disturbed joint. Stop the supply before correcting a leak.
- Adjust and prove While refilling, use the adjustment screw: clockwise raises water, counterclockwise lowers it. Follow the toilet's specified water line; for this installation Fluidmaster recommends about 1/2 inch below the overflow. Flush and recheck. Never set water into the overflow.
05 / VERIFY THE RESULT
Check operation and every disturbed joint.
- Dry the joints and check for moisture during refill and several flushes.
- The fill valve shuts off, the tank remains below the overflow, and the float moves freely.
- For a low bowl level, check refill-hose routing and the tank setting.
- For no fill or continuous fill, isolate the supply and follow the exact 400A cap-cleaning instructions; never open the cap while pressurized.
- If it refills by itself, test the flapper/flush valve for leakage instead of raising the tank level.
KNOW YOUR LIMITS
Stop and get qualified help when…
- Shutoff valve will not close, starts leaking, or requires force
- Cracked tank or damaged threaded connections
- C.L. height and lid clearance cannot both be achieved
- Leak remains with correctly seated seals and gentle tightening
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MANUFACTURER REFERENCES
Sources & scope.
Written instructions use the manufacturer references below. The 3D illustration is not a manufacturer certification or a substitute for the instructions supplied with your exact model.
- Fluidmaster 400A installation and troubleshooting
- Fluidmaster 400A quick-install booklet
- Fluidmaster 400A model guidance and exclusions
- Fluidmaster PRO45C instructions: captive connector seals and shank washer
Checked October 11, 2026. These are information links, not affiliate links. We do not sell tools or parts.