"just when we cleaned the trim there was a hollow spot inside..."
When you say cleaned the trim, do you mean the flash or cutting off the sprue? If it was the sprue revealing a void or air pocket then your feeder sprue might not be big enough to allow for shrinkeage as the lead is cooling. Have you sucessfully used these same molds before? If its the flash on the bottom of the cavity that would be unusual unless you are pouring in a way thats not allowing the air to come out as the lead goes in or youre running way too hot which is usually what flash indicates.
8 OZ is not a lot of lead. I pour up to 20 oz with no problems at all but the "feeder sprue is nice and large on those. What temp are you running your lead? . If its turning purple on top then youre way to hot. Pure melts at 621, but run it at about 700-750 . It sounds like youre using almost pure, pipe &trim usually is very soft and pure except for the stuff that floats to the top. Try also Fluxing. I use a special flux specifically made for lead, but you can use beeswax too. That powdery stuff(scum) needs to foat to the top and be skimmed off. Should look like glass on top when fluxed properly.
Also if youre sinkers are breaking(crumbling) as you dump them out,then youre opening the mold too soon &or running too hot.
"I poured the lead not to slow but never fast... I sometimes get a layered sinker in the begining..."
Thats normal as the mold is cold on the first cast,but then it should stop, esp on large sinkers.
Make sure you go to
www.do-itmolds.com (the link is not working right now but that is the right one) They have some very good info on there, and troubleshooting tips. They should have all the answers you need.