Have you tried tightening first?

I notice my valves need a little tweak every few years. I pick up a slow leak with my halide detector and I can stop it by tightening the cores a tiny bit. The ones I have use rubber seals and I guess they just creep over several years

ETA also I’ve noticed there is varying quality in cores. The ones I’ve received with fitting kits on eBay have tended to develop leaks faster than those I buy from the shelf at Autozone…I think the brand is something like AC Pro. Their cheap but they aren’t bottom of the bucket like the ones that come in free with the schrader tool I order direct from China.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles