Low flow from Kohler K15160 spray kitchen sink faucet. If I take off the spray handle I can fill a jug through the hose alone (including the filter screen) in 15 seconds while with handle on the same volume takes 55 seconds. The reverse flow prevention flap seems to be functional.

will water pressure remain low as long as spray head in in faucet body? have you checked under sink to insure no kinks or restrictions on line. check that counter weight on hose is secure and and not obstructing line. Call Kohler for more help.

yes any bend not made properly will affect the flow also makes a week spot.

Check to be sure the hose isn't kinked somewhere below the faucet. If there aren't any kinks there is likely debris stuck behind the nozle screen, remove it and clean it- or your handle mechanism is broken. Replacement handles and hoses are fairly inexpensive if you need one they're easy to find at most big hardware stores or on Amazon

Generally the sprayer trigger assembly will unscrew from the hose. You can then point the hose toward the sink and turn on the
water. It should flow freely through the hose when either the hot or cold is on. Generally there is a aerator on the spray head. You can access it by unscrewing the tip of the sprayer assembly and clean out any mineral deposits. There may also be an inline type filter in the handle where it screws onto the hose that can be removed and cleaned. If this doesn't help I would guess the on-off triggering assembly is broken and you would have to replace the sprayer head.Hope this helps.

You will need to replace the cartridge in the faucet

One thing you could try is to remove the aerator at the end of the spout on the faucet, Turn water on and see if pressure is any better. If so the aerator is got something in it restricting the flow.
You also can remove the dime shaped flat in the aerator to increase water flow. It looks like a dime with a hole about 1/8" in center to restrict the flow to about 1.6 gpm. Remove it and reinstall aerator and check water flow

Most of the times when cold and hot side of faucet.It would be the first thin to check being you have good water pressure at other faucets. They catch debris and the flow slows down from the faucet's. Check out supply lines and faucet for a blockage if this doesn't take care of the problem.

Check out this web sire for a parts breakdown of parts in faucet. Check out installation and rough in sheet for information on faucet below.
http://www.needplumbingsupplies.com/Kohler-Kitchen-Sink-Faucets.asp?id=3628

How do I take my faucet apart , I have a leak when I change it back and forth between spray and stream ?

Could be a blockage in the valves and check any filters any where in line.some sinks in kitchen have a filter under sink and check valve to make sure they are opened all the way.

Remove the air-irater at the end of the faucet where the water comes out. That is where 99% of clog are on faucets. If that does not do it then you need to replace the cartridge.

Good Luck and please rate 4 thumbs up for the free answer.

Your problem is a simple one.

1. Turn off water to faucet.
2. Place a towel(s) in the base of the sink(s) to prevent any parts from going down drain.
   
3. Remove the handle to the faucet. (single handle only)
   
4. If not a single handle then remove spout from faucet.
5. If a single handle remove cap carefully and any loose parts.
6. Gently turn spout side to side while lifting to remove.
7. You will see a hole with a very small valve where the water comes out to the spout on the base of the faucet.

1. Remove this valve (which is a backflow preventer) and replace.
2. Reassemble and test.
   

Problem solved.