Every faucet you will ever own uses one of four internal valve designs. Knowing which one you have determines the repair.
1
Compression (washer) faucets
The oldest design: two handles, and each one screws a rubber washer down against a brass seat to stop the water.
How it fails: the washer hardens and cracks, or the seat gets pitted by scale.
How we repair it: shut off supply, pull the stem, replace the washer and O-rings, then resurface or replace the valve seat with a seat wrench.
Cheapest parts of any faucet type, but also the design that drips most often. Houston scale eats the seats.
2
Ball faucets
A single handle rotating a slotted metal or plastic ball over spring-loaded rubber seats. Delta made this design famous in older single-handle kitchen faucets.
How it fails: the springs and rubber seats wear out, causing drips from the spout, or the O-rings wear, causing leaks at the base.
How we repair it: a rebuild kit that replaces the seats, springs, cam, and O-rings in one visit. If the ball itself is scored, it gets replaced too.
3
Cartridge faucets
A self-contained cylindrical cartridge controls flow and temperature. This is the most common design in American homes, and the entire repair usually comes down to one part.
How it fails: internal seals wear or scale binds the cartridge. Symptoms are dripping, a stiff or loose handle, or temperature that wanders.
How we repair it: pull the retaining clip, extract the old cartridge, grease and seat the new one, then index it so hot is actually hot.
Scaled Moen cartridges frequently seize in the valve body and need a puller. This is where DIY attempts turn into broken valve bodies.
4
Ceramic disc faucets
Two polished ceramic discs slide over each other to control flow. This is the most durable technology made, and it is what you will find in most current Kohler, Grohe, Kraus, and higher-end Delta and Moen faucets.
How it fails: rarely, but grit or scale between the discs can crack or score them, and the inlet seals under the disc assembly wear.
How we repair it: clean the inlet ports, replace the seals, or replace the disc cylinder.
One caution: ceramic disc faucets should never be forced closed. That is how discs crack.