On a modern tyre you should have 28-32 psi in the rear tyre...Many tyres have the correct pressure marked on the sidewall...I put in a little more when the bike is loaded up for touring...Ian
Also if you are ripping out the valve core ? you need to leave the lock nut lose and not tighten it up against the rim. I always leave a 1/2" gap between nut and rim this allows for movement of tyre and also you can spot when the tyre has crept as core will be at a angle, when this happens just deflate break bead and turn tyre to correct core then re-inflate ..
