when I unscrew the inner to pump it up

seems to seal when I screw it up again but may have a slow leak and I don’t really trust it

can you replace the valve cores only?

thanks

yup
Core

Best check the rubber hasn’t split where the valve enters / exits the rim. Had a similar slow leak and realised there was a slight split that began to leak when tightening down the screw on nut.

Presta valves have a little leak I find when you unscrew them, but all sealed after screwed back up.

If it’s a removable core valve, make sure the core is firmly screwed in place unless you want to remove the core. Has flat sides you can do it up with a mini spanner.

Then, use plenty of sealant generally and it will all seal up.