Loower looper keeps braking.

Could be that since it has set for years without use or maintenance, the old oil has dried out and the motor is straining to run the machine without sufficient lubrication. Trying to run the machine in its present condition could damage the motor. Your machine very likely just needs a thorough oiling (1-2 drops of fresh good quality serger oil) every place that metal rubs on metal. (Sergers usually take a lighter-weight clear oil--different from what sewing machines would use.) You should probably throw out the oil that came with the machine. It is probably old and should not be used in your machine.

A serger requires much more frequent oiling (the moving parts behind the front door, ie the looper mechanics, etc.) than a sewing machine. In addition, about every year I take the bottom off my serger and lube the moving metal parts inside with Tri-Flow Synthetic Lube. It has a whole different sound after getting some TLC!

To keep your serger operating for years, be sure to regularly clean and apply a couple drops of lube as recommended in the owner's manual (maintenance section).

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwj1woi47IzQAhUCwmMKHVVPCBQQFggtMAA&url=https%3A%2F%2Fwww.brother-usa.com%2FModelDocuments%2FConsumer%2FUsers%2520Manual%2FUM_PL_2100_EN_238.PDF&usg=AFQjCNF99J5PVNoFcHjv5FJcR-6eyNqTNg&cad=rja

Search the internet. There are web sites and videos that show machine timing, although adjusting a serger may a bit more challenging.

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I take it this is a serger from the fact you have a looper. Try changing which looper thread is on top when you thread it. Mine jams if they're not done just right. BTW, the easiest way to thread a serger once you've got it once is to tie the new thread to the old and pull it through.

Hope this helps.

Has it been serviced at all during your ownership of the serger?

The loopers can be knocked a little out of timing resulting in the skipped stiches on the heavy weight fleece occassionally. It sounds like you have tried all the usual fixes and know what you are doing.

Only other thing that might cause skipped stitches could be using a sharp needle rather than a ball point on the fleece. Otherwise maybe time for a trip to the shop??

I will make this easy first always thread the machine upper looper then lower looper then the needles if a thread breaks especially if it was one of the loopers, make sure to pull the needle thread out before hand then rethread. as for the breakages look to see if the loopers are hitting each other when they cross and see if the needles are hitting the loopers when they go together either of these things will cause problems if so it will need to be looked at by a tech

Your serger should be color coded for threading. Just do one at a time either left to right or right to left with the cones. And you should be all set.

tight for thread in loower looper

You will need to buy a new looper from a dealer with a spare parts account for this brand.
It is tricky to set up the loopers without a service manual, so best suggest you also have them install and service the machine at the same time or you risk breaking the new one if it is even slightly out of alignment.
You may also find that parts availability at this time of year will be restricted as many businesses and suppliers close for several weeks.

Stretch fabric is made for using the differential feed on your machine, sergers love stretchy things. Try widening the stitch you are using and turn your differential feed down one notch to allow the machine to get a good grip on the fabric, you may need to bump the differential feed up more if it still doesn't stitch nice, keep trying on a piece of scrap fabric of the same as you are using until you are happy with the stitch.Another solution is to put some lightweight tissue paper under the seams and then tear it off after you have sewn the seams, this works but will dull your blades over time.

You can get sewing machine manuals online at sewingmanuals.com. I didn't see the superlock 2500 but there were other superlocks. They may thread the same way.