I used to hold the valve stem with a chuck type tap wrench, I used to same tap wrench to grind the valves in BTW, I'd then just pull against the valve whilst filling the inlet or exhaust port up with petrol, nothing finds leaks like petrol, just using hand pressure if I could spot a weep in the combustion chamber I'd regrind it until I didn't. With petrol the weep looked like vapour in the air, very easy to spot.