Any tips on capping off angle stop

I bought a 3/8" compression cap to go on a angle stop.
Put a libleral application on pipe tope on the threads and the cap wrenched on easily till it hit the bottomed out - end of the angle stop body.

Still leaks water - slight drip if the valve is opened.
I am capping it for this very same reason (even though the valve can be closed, if someone turns it by mistake.....I don't want any water coming out).

Any advice...

cap

You have to make a solid rubber washer the size of the cap and put it into the cap before you tighten it.

Take a nipple and grind an end to a thin taper and use it for a gasket maker. You will make a round rubber washer for your cap. I think you use a 3/8" nipple.

Or flatten, fold, and solder a piece of 3/8 copper to seal the end and put it in the compression fitting normally.

you still need to use a ferrule when using a compression cap

Hmm - didn't think of using of the ferrule....

With ferrule, does it cap tight without the suggestions of the DIY washers ?