Clutch leaking can it be the slave cylinder o ring?

[gertsdsbs]

So, my clutch is fine during the day i can bleed it and it will be fine for a couple of hours but when i leave it over night it losses pressure. I removed the o-ring in the slave cylinder where the the bleeder valve is at at and the o ring is worn out right in the middle all around. so can this be the problem? it's a 1986 ford ranger 2.9 4x4 6 cylinder manual transmission. THANKS!