Kitchen and bathroom Delta faucets having two handles are stem-type faucets that come with a valve seat made of neoprene which fits into the faucet under the cartridge-like stem.
This is not the same as a real cartridge-style faucet which looks a lot like the stem faucet without the springs and valve seats.
The stem-style faucet allows a neoprene valve seat and spring to be fit into the opening of the water inlet in the valve body. A bonnet nut holds the stem in the body above the valve seat and spring. The handle is now attached atop the stem.
When there is a leak in faucets such as this, the repair involves the use of a new neoprene valve seat and spring.
But there might also be need for replacement of the entire stem if the handle stiffens when the faucet is turned on.
This is if a leak is observed around the handle when turning on the faucet or if the leak persists even after the replacement of the seat and spring.
Delta Faucet Repair: Establish the leaking side
Feel the water dripping from the faucet.
If warm, the hot side is most likely the site of the leak. If cold, the cold side is most likely the site of the leak.
With this, you'll know the side that needs to be repaired first. However, there's a possibility that the two sides are leaking.
Usually, the valves on the hot side are worn faster because of how the neoprene is hardened by the hot water.
A lot of people prefer to replace the two valves at once.
Turn off the water
Check underneath the sink to find the shutoff valve of the faucet side you're to fix, be it hot or cold.
There may be a little football-shaped handle or lever or a knob on the valve.
Rotate that handle in the clockwise direction till it stops.
Ensure the water is turned off by opening the hot or cold faucet as applicable.
If the sink does not have any shutoff valves underneath, turn off the shutoff valve of the main supply to the whole house and turn on the two faucet sides to release any pressure in the supply lines.
Take off the handle of the faucet
Use a screwdriver to remove the screw atop the handle to loosen it or use an Allen wrench to unscrew a setscrew on the side of the handle to loosen it. Pull the handle up to take it out of the faucet stem.
Note: to reach the screw on the handle, you might need to take off a decorative cap.
Unfasten the bonnet nut
With an adjustable wrench or tongue-and-groove-pliers, loosen the bonnet nut that holds the faucet stem in place by turning it in the anti-clockwise direction.
Completely unthread the nut and take it out of the stem.
Take out the stem
Take note of the placement of the stem before taking it out. It must be reinstalled the exact same way in order to have the faucet work correctly.
There are usually one or more tabs in stems that fit into notches in the housing of the stem to ensure the stem is aligned.
However, it can still be gotten backwards. If you wish, you could take a picture of the stem.
Pull out the stem to get it out of its housing. If the need arises, use pliers to grip the spindle atop the stem.
Take out the valve seat and spring
Look into the housing of the stem to find the neoprene valve seat. You'll see it in a hole at the rear end of the faucet housing.
It is a small, black, rubber cap with a hole on top of it. Inside and underneath the seat is a little metal spring.
Insert a small screwdriver or an Allen wrench into the seat's hole to take off the seat and spring. Both parts will be pulled up at the same time.
Take note of the placement of the spring inside the valve seat. It's diameter at the top should be a bit smaller than at the bottom (because it fits into the seat at that point).
Fit in a new spring and seat
Place the new seat and spring on the end of a screwdriver or an Allen wrench in the exact placement as the old seat and spring.
With a wrench or screwdriver, carefully insert the spring and seat into the hole in the housing of the stem.
They have to get to the bottom of the hole.
Take out the wrench or screwdriver and check to see if the positioning of the valve seat is correct.
Replace the stem
Use a little amount of plumber's grease to lubricate the O-ring at the base of the valve stem.
You may also change the O-ring (it often comes with stem-and-spring-kits) or the stem completely if you so wish.
Place the stem in its housing ensuring that the tabs on the stem align with the slots in the housing.
Ensure that the stem is pushed all the way to the end.
Replace the bonnet nut and use an adjustable wrench or pliers to tighten it.
To avoid damaging the stem housing, the nut should not be overtightened.
Delta Faucet Repair: Re-fit the handle
Open the shutoff valve underneath the sink to turn the water back on.
Do this by turning the valve in the anti-clockwise direction till it stops.
Check the faucet stem to see if there are leaks.
If there are leaks, keep tightening the bonnet nut till it stops but still try not to overtighten.
Attach the handle to the stem and use the setscrew or screw to hold it in place.