Best way to find slow leak in pcp

Remove the bottle and submerge the valve end in water. Look for bubbles.

Thurmond

Soapy water makes the bubblesl last longer.

Soapy or shampoo water works great for me.

If you can't find it with water or soapy water, take it to your local propane or natural gas supplier. They have a leak detect solution more persistent than soapy water that will show extremely small and slow leaks.

you can always go pre-mix that works like a charm. Dip finger in solution, and apply where you suspect the leak is:

https://www.amazon.com/Gazillion-Bubbles-1-Liter-Solution/dp/B001LO9TS8