Delta 9178 Faucet Leaking: Which Part to Replace
Delta's own manual for the 9178-DST: tighten the bonnet nut for a leak under the handle, then replace the RP50587 cartridge. What a drip after shutoff means.
By Askento Editorial Team · 3 min read · Oct 14, 2026
Written with AI assistance, grounded in cited sources and checked by automated quality gates. How we create our content
General information only. This article may include AI-assisted content. While we aim for accuracy, verify important details before acting on them. Affiliate disclosure: This page may contain affiliate links. We may earn a commission at no extra cost to you.
The short answer, from Delta's own manual for the 9178-DST: if it leaks from under the handle, tighten the bonnet nut first. If it still leaks, or if water drips from the spout, replace the valve cartridge, Delta part RP50587.
Everything below comes from Delta's installation and care manual for its high-rise pull-down kitchen and bar faucets (document 85214, Rev. G), which covers the 9178-DST. It is linked in the sources.
Where Is It Leaking?
| Where the water comes from | What Delta says to do | Part |
|---|---|---|
| Under the handle | Remove the handle and cap, and tighten the bonnet nut with a wrench. If it still leaks, shut off the water and replace the valve cartridge. | RP50587 (valve assembly) |
| From the spout, continuously | Shut off the water and replace the valve cartridge. | RP50587 |
| From the spout, briefly after shutoff | Nothing. Delta says a short drip after shutoff is normal, caused by the long flexible hose. | — |
| Low flow rather than a leak | Clean the aerator, or the screen inside the spray head. | — |
After any repair, Delta's instruction is the same: tighten the bonnet nut securely with a wrench. The manual warns that a loose bonnet nut can cause water damage.
Replacing the RP50587 Cartridge
The manual doesn't list the removal steps one by one. This order follows its numbering, where the handle (4), cap (5) and bonnet nut (6) sit over the cartridge (7).
- Shut off the water supplies under the sink.
- Remove the handle and the cap beneath it to reach the bonnet nut.
- Unscrew the bonnet nut and lift out the old cartridge.
- Fit the new RP50587 cartridge, refit the bonnet nut and tighten it securely with a wrench.
- Refit the cap and handle, turn the water back on and check for leaks.
Don't try to open the old cartridge. Delta says there are no repairable parts inside, so the fix is always a new cartridge.
A Leak at the Base or Under the Sink
Delta's manual covers leaks from under the handle and from the spout, but not water at the base of the faucet or under the sink. Before buying anything, find where the water actually starts: dry the faucet and the cabinet completely, run the water, and watch. A leak under the handle can run down the body and look like a base leak. If the water is clearly coming from somewhere the manual doesn't cover, contact Delta with your model number rather than guessing at parts.
Slow Flow Instead of a Leak
Delta's manual gives two fixes: remove and clean the aerator (some models need the wrench supplied with the faucet), or unscrew the hose from the spray head and clean the screen just inside it. Put the screen back: Delta warns that leaving it out can damage internal parts.
Related
- How to fix a leaky faucet: other brands and faucet types
- Best faucet repair kits: brand-specific cartridges and kits
Frequently Asked Questions
- Why is my Delta 9178 leaking from under the handle?
- Delta's manual for the 9178-DST says to remove the handle and cap and use a wrench to make sure the bonnet nut is tight. If it still leaks, shut off the water supplies and replace the valve cartridge, then tighten the bonnet nut securely with a wrench.
- What cartridge does the Delta 9178 use?
- In Delta's parts list for the 9178-DST and the other faucets covered by the same manual, the valve cartridge is part RP50587, listed as the valve assembly. Delta says not to take it apart: there are no repairable parts inside.
- Is it normal for my Delta pull-down faucet to drip after I turn it off?
- A little, yes. Delta's manual says a small amount of water may run out of the spout or drip for a very short time after shutoff, caused by the long flexible hose. A drip that keeps going is a leak from the spout outlet, and Delta's fix for that is a new valve cartridge.
- Why is my Delta 9178 so slow?
- Delta's manual says to remove and clean the aerator (some models need the supplied wrench), or unscrew the hose from the spray head and clean the debris screen just inside it. Put the screen back afterwards: Delta warns that leaving it out can damage internal parts.
On this page
Browse topics