My mini-pump has a screw-on head (as I guess most of them do) and several times when removing the pump from the valve, it's taken the valve core out with it.
The last time this happened to me on the road, I tried screwing the core in tighter and it simply sheered in half, leaving me with broken valve core stuck in the valve shaft. There seems to be a fairly narrow range where the core is tight enough to stay in place, and not so tight that it breaks.
Are there any reliable ways to prevent this, or any adapters or attachments for mini-pumps that do not screw onto the valve core? I am using tubeless tyres, so I can't use a valve without an integrated core.