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Make sure you put the valve spring seats on the stems BEFORE putting the stem seals on. The spring seats will not pass over the stem seal, and if you try taking the seal off to fit the seats, you ALWAYS damage it getting it off. Don't ask, seen it happen too many times!:-[
Did you remember to tap the top of the springs with a hammer, makes sure the collets are engaged.I also used to fill the tracts with petrol, if you can see any seepage past the valve seat you've not lapped them enough. Petrol seems to be very effective at finding any imperfections in that area.
I used to test mine by using a tap wrench, tap as in tap and die set not the kitchen tap I used the chuck type tap wrench, I'd insert the valve with some grinding paste on the seat and then attach the tap wrench to the stem, I found that this way I could pull the valve onto the seat much better than using a good damn awful sucker on a stick. When I wanted to test with petrol I'd just pull hard on the tap wrench, probably less pressure than the springs so if it didn't leak then it was unlikely to leak with the springs on. Similarly I could test the valve seats before removing the springs to see which ones were leaking, if you watch the valve head you can spot the petrol vapour coming out if it was a slight leak or dripping out if the seat was badly burned.