How to Adjust a Korky Fill Valve: Model 528 and Others

To adjust a Korky fill valve, shut off the water, twist the valve top counterclockwise to release its height lock, slide it until the stamped WATER LEVEL mark sits about 1/2 inch below the top of the overflow tube, then twist it clockwise to lock. There is no float screw to turn.

Which Korky fill valve do you have? Start with the cap color

Cap color separates the 4 common Korky valves: white is a 528, silver a 528MP, black a 528X and blue a 528T, according to Korky’s fill valve replacement guide. The drawing on the 2026 528MP sheet shows “528” molded beside the C/L mark on a silver-top valve, so a molded 528 does not prove you own the plain 528.

Korky’s support FAQ says all of its fill valves use an internal float, so no row has a float setting: the height adjustment is your water level control.

Read the last column first. A 528 or 528T has 1 adjustment, height, so those valves offer no way to send more water to the bowl. A 528MP, 528X or 528GT has 2, and they do separate jobs: height sets the tank, the dial sets the bowl. Korky’s 4010 kit ships the 528 and its 4010MP kit ships the 528MP. A 528Z also needs its chain checked, because too much slack stops the red tilt cup from tilting and too little stops the flapper from lifting. An empty tank on a 528Z can be the valve signaling a leak by design: hold the handle or pull the red chain for 5 seconds to refill, then repair the leak.

How do you adjust the height on a Korky? 9 steps with turn direction

You adjust height by moving the valve body itself, and the 528 sheet (LIT024) and 528MP/528X sheet (LIT148) both lock it with the valve out of the tank.

1. Close the supply stop, then flush and hold the handle to drain the tank. Sponge out what remains.
2. Unclip the refill tube from the overflow. Set a bucket under the tank and disconnect the supply line, which still holds water. Remove the mounting nut, by hand or with an adjustable wrench, and lift the valve out.
3. Hold the lower shaft and twist the top counterclockwise. This releases the height lock.
4. Slide the top up to full extension, so you are setting the height from above on the next step.
5. Stand the valve in the tank hole and push down on the top until the stamped WATER LEVEL mark is about 1/2 inch below the overflow top. This is the actual adjustment, because the float rides inside the valve and its shutoff point travels with the mark.
6. Lift the valve out, hold the shaft and twist the top clockwise until it snaps. The rib lines up with the peg when locked, and the valve rises slightly.
7. Set the valve back at 45 degrees from the back of the tank, giving the refill tube a straight path. Thread the nut on beveled side up, hand tight only.
8. Check that the C/L mark on the cover is 1 inch above the overflow top. If it is not, return to step 3.
9. Clip the refill tube to the overflow lip, reconnect the supply by hand with no sealant or tape, and open the stop slowly. Water from the refill tube, valve top and valve base is normal during fill.

Pushing the valve down lowers the tank level; raising it does the reverse. Korky’s guide names both failure points: too high and the valve may not shut off or may overflow, too low and the flush turns weak or incomplete. No sheet describes re-setting the valve with the nut still tight, and Korky publishes no turn count for the height lock. The 1/8 turn you may read about belongs to the cap. One trap on paper: the 2026 528MP sheet’s shutoff note says to revisit step 3b, but height is step 4b on that revision.

Where should the water sit in a Korky tank? 1/2 inch below the overflow, not 1 inch

Water should sit about 1/2 inch below the top of the overflow tube, a figure given on the 528 sheet, the 528MP product page and Korky’s noise guide, and the same water line used in our guide to toilet tank parts.

The 1 inch in Korky’s documents is a different height: the critical level (C/L) mark sits 1 inch above the overflow, which the 528 sheet labels a code requirement. Add the two and the marks are 1-1/2 inches apart on the valve. Setting the water mark therefore places the C/L mark for you, so step 8 is a check and not a second adjustment. Local plumbing amendments vary.

On a tower-style flush valve, the 528PRO sheet says to match the valve’s water line to the stain line in the tank. For a fast look without a ruler, Korky’s noise guide says the bottom of the cap should line up with the overflow tube.

How do you set the refill dial on a 528MP, 528X or 528GT?

Turn the dial clockwise for more water to the bowl and counterclockwise for less. The dial splits incoming flow, and the 528X sheet says the open position directs 40% to the bowl and 60% to the tank. Height still decides where the tank stops.

• Flush, wait 2 minutes for the bowl to settle, and mark the bowl’s water line in pencil.
• Flush again and watch the bowl as the valve shuts off.
• Within 1/2 inch of the mark: Korky says leave it.
• Above the mark: turn the dial counterclockwise. Below the mark: turn it clockwise. Repeat until the bowl reaches the mark as the tank finishes filling.

The 528GT and 528PRO sheets add a numbered key: start with the dial fully closed (counterclockwise), set the key from the toilet-model chart, then turn clockwise until the dial touches it. Keys 1 to 8 give 12, 20, 22.5, 25, 30, 32.5, 35 and 37.5% refill. The charts are not interchangeable. A TOTO Drake 743S is key 5 on the 528PRO chart and key 3 on the 528GT chart, so use the chart printed for your valve. Toilets absent from the chart use the pencil test instead.

Why is my Korky fill valve hissing, running on or filling slowly?

A constant hiss usually means water is escaping into the bowl, and Korky’s noise guide names a worn flapper or flush valve seal as the most frequent cause, ahead of the fill valve.

If the level holds steady but the toilet still runs, the flapper and chain checks in our running-toilet diagnosis come next. Refill-time and pressure benchmarks are in our slow-filling toilet guide. Two results mean stop and hire out: little or no flow in the bucket test, where Korky says a licensed plumber should look for a blockage in the line or incoming supply, and banging on other fixtures, where you should ask for a water pressure check and a water hammer arrestor.

How do you clean a Korky valve? Cap off, strainer out, warm rinse

Cleaning a Korky means removing the cap and pulling the strainer, then rinsing both in warm water, following Korky’s 13-step service guide, condensed here.

1. Close the stop and flush to empty the tank. Take the refill tube off the valve cover.
2. Lift the cover off by its tab, then pinch the float arms together and remove the float, marked with an X.
3. Grip the valve body and turn the cap left 1/8 turn to remove it, then rinse it. Both moves run counterclockwise, so a loose grip on the body also releases your height lock.
4. Pull the supply tube straight out, pull the white strainer from its bottom end and rinse it.
5. Reassemble in reverse, turning the cap clockwise. Tap the X to confirm the float moves freely without scraping.
6. Clip the refill tube to the overflow lip and turn the valve body clockwise to confirm it is still locked.

Past 5 years, or with a damaged part, Korky favors replacement over service. A worn cap can also be swapped alone with the R528 replacement cap assembly, whose sheet uses the same 1/8 turn.

How is adjusting a Korky different from a Fluidmaster 400A?

A Korky has 1 water level control and a Fluidmaster 400A (part 4-743, rev. 6) has 2, so instructions written for one fail on the other. Guides that tell you to turn a screw on a Korky are describing the Fluidmaster’s float cup.

Both makers land on the same 1/2 inch water line and 1 inch C/L mark, so only the method changes. If the valve keeps failing, our page on signs of a bad fill valve and the replacement steps covers when to swap it.