The cap isnt 5 as far as I can tell Greaves now cap leathercraft at 6.
I was at Leathercraft Lv 4 and quickly got to Lv 6 within 10 synths or so.
Ive synthed 12 after reaching Lv 6 leathercraft with no further leathercraft skill ups. Ive now stopped synthing as I have reached the Smithing cap (33).