Ok saw a few people chatting about these on facebook so thought id quickly do a guide on how to make a water system leak tester. You will require a tyre valve and an old header tank cap. Takes all of 2 minutes to make 
step 1, get valve

step 2, get header tank cap

step 3, stick flat blade screwdriver under the coloured part of the cap and twist until the cap comes off and the gubbins fall out

step 4, add a little sealant to the valve and pull through the hole.

job done
step 1, get valve

step 2, get header tank cap

step 3, stick flat blade screwdriver under the coloured part of the cap and twist until the cap comes off and the gubbins fall out

step 4, add a little sealant to the valve and pull through the hole.

job done