Q: How do I tell if my stop valve is compression or flare?
A: Look closely at the copper pipe where it meets the valve. If the pipe slides straight into the nut with no flared edge, it’s a compression-type valve. You’ll see a brass ring (ferrule) under the nut. If the copper line ends with a small outward flare that seats inside the valve, it’s a flare-type valve. Most homes built in the 1980s used compression valves.
Q: Which cone gasket seals a flexible fill line?
A: Most toilet supply lines use a ⅜-inch compression cone washer. The narrow end points toward the valve and seats inside the outlet. Avoid stacking gaskets; one correct washer is enough. Choose rubber or nylon, rated for potable water. You can find them at any hardware store or online at Home Depot or Lowe’s.
Q: I’m scared to replace the valve. What should I know?
A: If the valve was installed in 1987, the compression nut may be fused to the copper line. Turning it can crack the ferrule and cause leaks. If you’re nervous, leave the old valve in place and only replace the flex line and gasket. For full valve replacement, shut off the main water supply and use a compression sleeve puller to remove it safely.
Q: The Fluidmaster 400A fill valve gasket leaks. Can I add a flat washer?
A: No. Adding another washer often worsens the seal. The rubber gasket belongs inside the tank, not under it. If the supplied gasket feels too small, replace it with the Fluidmaster 242 universal seal kit. This thicker gasket fits older tanks and provides a watertight seal without overtightening.
Q: How can I prevent ceiling damage from future leaks?
A: Always dry connections thoroughly after repairs, then check for drips with a tissue. Leave a towel under the valve overnight to monitor moisture. Small leaks can cause major damage on upper floors, so consider installing a smart leak sensor near the valve for peace of mind.
I’m Chris Mayer, writing for Plumbing 101.