If you have found the wonderful issue of brand new joker valves leaking, I might have a solution for you.
The new valves are a horrible design with three sided oneway valve that if left sitting on a shop shelf will invariably be warped by the time you get it and the three sides of the valve won’t match up together.
So the Cure!!!
Cut, from the old one a ring of rubber, by cutting through from side to side up near the top of the valve. Push this onto the new valve so that it compresses, just a little the closure and apply a little grease to the valve (rubber grease is best).
Then insert and install as per usual. The rubber band might not last for ever but it will put more pressure on the newly inserted valve and reshape it so that it will seal. Nice little trick that has worked a number of times for me now.
Because those joker valves are no ones joke.
BTW, have you ever noticed how the instructor videos are always with perfectly clean, brand new, never used heads? makes it look so easy and sanitary!!!