Toyota does (at least did) have a problem with brake lines.
When I replaced the rear brake lines on my 2004 4Runner this year, I noticed the section of lines over the rear differential AND at the junction under the driver's door were EXTREMELY CORRODED.
The lines have some sort of coating on most of the line, but a couple inches at the END of the lines are NOT coated. They were extremely rusted, requiring replacement of all affected lines from that junction back and the lines on the rear diff.
The coated sections of line looked brand new. The uncoated sections were mostly destroyed by rust at EVERY location.
I'd put money on yours being the same way and DEFINITELY SUGGEST you have at a minimum, the junction near the driver's door checked.
Yes, it is a Tundra, not 4Runner, but you'd be surprised how similar they are.
