Six years ago I bought Bedford 700 x 19 tires and tubes from Lucas. The tires are fine but the tubes leaked at various rates. I then ordered tubes from Coker which gave much better results. One of my tires does leak slowly even now. In examining the tubes I found they all leaked at the point that the tube and the stem are joined/vulcanised together. You may have to exersize a ‘trial and error’ process to get a set that will hold. Good tubes still have a slow leak factor we don’t see in tubless tires. The average ‘good’ tire can leak a pound per week!