Re: How do you tie monofiliment line to 10 color lead filled line
...somehow the swivel parted and that was the end of that. Better swivels from now on..
On swivels: I only use barrel swivels for any swivel that is going to go through the guides and wind onto the reel, and my leaders are almost exclusively 14# Seaguar clear fluorocarbon.
The swivels are not the "power swivel" style but are the ones with the wire twisted 'outside" the body. The ball bearing swivels are too bulky and do not wind through the guides well. I predominantly troll for walleye and most often landlocked atlantic salmon and usually on Lake Champlain. A #12 barrel swivel has a 26lb rating which is what I mostly employ. A #10 is rated at 31lbs and gets used when I can't get 12s. I terminate all my leaders with a #1 duolock snap (or a #2 if 1s are not available). The #1s are rated 20lbs and the #2s are 25lbs.
The point is that I am going to answer Original Poster's question by telling you what I do. The information above and below will explain to you why those things work.
My leadcore setups are 8'6" Okuma Magda medium 'glass trolling rods. These are stiff enough to handle the leadcore while still adding some compliance to help keep fish stuck on. These hold Magda Pro 30 line counter reels (redundant, I know) spooled with 10 colors of 27# leadcore
(I used to use 18# but I had the drag set hard on a running fish and it broke off at the rod tip losing 6 colors along with a mega fish so I upsized) on top of 20# mono. (The mono backer isn't so much for backer as it is to keep the lead from slipping on the spool)
Here's the setup: 20yds 20lb mono stretched on the spool with 50 yards of 30# Powerpro braid backer and 10 colors 27# leadcore over that. Then 30 feet of 20lb clear mono "shock leader" is tied to the leadcore, then a #12 swivel with 6 feet of 14lb fluorocarbon leader is terminated with a #1 duolock snap ready for a spoon or a plug. The leader is the weakest link.
(I went to 14# fluorocarbon because I was using 10lb fluorocarbon and had fish break off. I was going to go to 12lb leader but I tested some borrowed 12lb fluorocarbon and found that its knot strength was only about 9lbs and big lake trout broke me off and I HATE losing tackle. No more break-offs!)
The Seaguar fluorocarbon leader is tied with a "trilene" knot to the swivel and then tied the 20lb to the swivel. After the swivel is tied on both sides I strip 6"+ from the leadcore end. Then I slide the 20lb into the dacron sheath of the leadcore. Holding it firmly, I tie a dbl overhand knot in the leadcore. I strip about 5" of lead out of the end leaving only the dacron sheath which is a very simple yet effective way to deal with it. If not doing the "shock leader" thingy then that dangly end of the leadcore should be tied using a palomar knot.