Perhaps the main valve is not dripping ( check by removing the jets) When you take the jets out you may find a tubular filter, you may also find that this has a non return valve fitted, this is put in back to front so that the water pressure opens it while spraying but it closes when not spraying. This stops the drip on the back stroke. These have a habit of furring up or getting a bit of fibre behind the ball which allows the dreaded drip. If everything is cleaned, possibly with a bit of acetic acid then with luck everything will be fine.
Bear in mind that jets do wear out and the hole gets progressively bigger, esp brass jets. If you can replace with stainless, they may be expensive but last many times longer. I tend to use plastic jets as I can obtain them from the local agricultural warehouse for 75-95 p each and last nearly as long as stainless.
Trevor