Malibue outdoor lights

It's a function of distance, and the Malibu web site states the limitations. Has the light output diminished recently?

The farther the dimmer.... that would be the cable/wire.

I had them in the basement for years unused, but with the 100ft cable maybe its normal since I tried another old transformer and its dim.

Do you mean the wire is old and corroded and new wire would help?

Yes.

In general wiring, there is always resistance in the wire so it is normal to be a little dimmer at the far end. A good system will have wire heavy enough to make that hardly or not noticeable at all.

But... you also have a connection at each light, after 35 years it's due for a tune up. Start by taking the first light and redoing the wire nuts at each connection, they are probably a little green by now. Cut off the old corroded wire and make a new connection with clean wire, new wire nuts, and dielectric grease. If you don't have any wire to spare you could try to simply tighten the wire nuts. If not, then replace the wire.

If the lamps had some type of built in clip on connectors it will have to be checked carefully, hopefully they aren't corroded and it wont require new lamp fixtures. If I were in that situation I'd probably solder the wire to the old lamp fixtures, but learning to solder in this situation probably isn't a good idea.

Transformers don't get weak, but they (and wire connections) might corrode. You'll need to do some troubleshooting. Yes, even copper corrodes over time so that may be what you need to replace. I have a "Made in China" transformer, going on 16 years outdoors. It stays dry inside a "birdhouse" I made specifically for it.

The actual transformer willnot get weak, it either works or does not work. The connections into and out of the transformer are another matter. They can corrode and drop the voltage.