Two light fixtures on three-way switch. Lights are dimmed.

Recently I noticed that my breaker was tripping when turning on my hall way light light. When this happened it would kill the whole run (attic light, living room light fixture, sun room fixture, hall way light fixture and front porch fixture). After following the lines and eliminating the options I found that I had an old black wire whose insulation had crumbled and touched the metal mount box (it was the front porch fixture). That particular fixture is operated in conjunction with the upstairs hall way light (it is an old duplex with separate apartment entrance at top of stairs) and is controlled by three-way switches, one at the bottom of the stairs and one at the top. So I have two fixtures operated together on a three-way. As I have an old brick house and the fixture is mounted on the front of an ornate column, the easiest way to rerun the wire to replace the crumbling wire was to drill through the masonry on the exterior of the brick facade and into the interior of the exterior wall to the switch box. This was accomplished without incident with outdoor Romex threaded and concealed in the grout line to the side of the column and into the back of the fixture box. So up to this point I have only replaced the line from the downstairs switch box to the outdoor fixture on the column. Now is where things get confusing. When replacing the new Romex to the downstairs switch, I connected the load (wire to fixture) to the black terminal and the red traveller to the top terminal above the black terminal. The black hot is to the terminal on the other side. The white neutrals are wire nutted together for return. Thought this would solve all my troubles. Now when I flip the lights on, all other lights on the run (attic, living room, sun room) work fine but the two fixtures tied to the three-way are dimmed. So I realized I have wired in series and need to parallel the line to the porch light. But upon inspection in the downstairs switch box I can't find another traveller to pigtail into to run a parallel line to the front porch fixture. I ran up to the attic to check that the the power was coming first to the upstairs switch, then to the upstairs light, then down the side wall to the downstairs switch, then to the front porch fixture. So it is. I have tried refiguring my replacement wiring on the downstairs switch (moving the black, traveler, and load) and still dimming. I have replaced both three-way switches thinking it was bad switches. Still dimming. I noticed some of the wires in the attic have become a meal to some rats who have chewed through some insulation. (Yikes, need to address that, but first want to get my lights to stay on bright!) so I am thinking that maybe that is the root of all my issues, but thought that that would cause a short and not a dim instead.

So, what am I missing? Feel I have covered my bases but obviously am missing something. Don't want to hire an electrician if all I am missing is connecting a simple miss-wire.

Any suggestions? Is this a really simple fix or am I up to something over my head?

What is the route that the wire takes from the panel to the last box?

Double check your connection?

Have you ringed out your runs?

This would be simple fix if only replaced the wire that was faulting? Unless it wasn't replaced with 12 gaged wire.

You said you tide something else into the lines?

It's also possible that one of your whites could be a hot line and was never tagged.
Although dimming seems like a ground not connected correctly.

You need to run the new cable from the porch fixture to the same spot it was before, not the nearest switch. I suspect it either went to the other switch or the other fixture.