Could be the o-ring on the pitless adapter blew out.
It would be nice if you could have a new o-ring standing by, but you don't know what pitless you have.
Valveman has posted that he can often find a matching o-ring for a pitless using a kit that has a bunch of o-rings.
https://www.zoro.com/o-ring-assortments/c/7463/ is Zoro's offerings.
Your local big amazing hardware store may have one that works.
I would coat my new pitless lightly o-ring with Molykote 111, because I have it. But if you are also going to use this on moving seals, you can get Dow No. 7 release compound and use it for both static and dynamic seals.
It would be nice if you could have a new o-ring standing by, but you don't know what pitless you have.
Valveman has posted that he can often find a matching o-ring for a pitless using a kit that has a bunch of o-rings.
https://www.zoro.com/o-ring-assortments/c/7463/ is Zoro's offerings.
Your local big amazing hardware store may have one that works.
I would coat my new pitless lightly o-ring with Molykote 111, because I have it. But if you are also going to use this on moving seals, you can get Dow No. 7 release compound and use it for both static and dynamic seals.