How close is too close

Why continue to worry about it? Rerouting your brake lines is not that difficult. It is something you can do yourself if you are at all handy. Check the size of your brake lines and you can buy just the steel tubing at any NAPA store, along with the couplers. Then you need to buy a DOUBLE flaring tool, which can run from about $20 on up, depending if it is made in China or in a decent country. DO NOT use a standard plumbing single flaring tool.
Once you use a tubing cutter to cut the brake lines, then you can add additional tubing to route the brake lines away from any heat source. Once you have everything rerouted and securely fastened, you will need to bleed your brakes and enjoy a permanent solution to your problem.
Naturally any competent garage can do the same thing if you do not want to do the work.

I've always had mixed feelings about the concept of wrapping things to protect from heat, especially in the case where the fluid is not moving. A wrap will only slow the transfer of heat, not dissipate it. OK, so it buys you time on a long grade, but how much time do you need. I would much rather see a heat shield. If the shield is big enough, areas of the shield will be away from the header and be able to dissipate the heat away. I would try to pick up some aluminum sheeting, as it is easy to bend and has good heat transfer characteristics. Its also less likely to rust.

Put a wrap of tape and if it burns then it getting hot. Along with what Wolfe stated. If you don't have headers then you should be ok but the tape will tell you.

Yes, the boiling point of FRESH brake fluid is over 400 degrees F.

But, as it absorbs moisture, the boiling point lowers to 286 degrees F (read the fine print on your can of brake fluid).

So absolutely, flush brake fluid every 2-3 years. More often in humid/wet climates.

And, wrapping the lines/insulating them might be a viable option to rerouting them.

The Thorley headers on my 454 caused several problems at first.

>Melted the casing on the speedo cable. It ran above the headers over to the transmission. I finally found some high heat resistant heater hose, that I cut open and covered the new speedo cable with.
> Never lost brakes but they got spongy so I covered the brake lines near the headers with fiberglas cloth.
> Melted the power cables at the starter eventhough there was a heat shield. I installed new cables and wrapped them with the heater hose. Over time (about 2 years) it got brittle so I had to replace it.
> Burnt spark plug wires. I tried a couple of sets but nothing worked until I bought a set with ceramic ends for the spark plug caps.

At the time we were going up several different 6 to 10% grades in 80 degree heat. Most of the grades were under 10 miles long but he worst one was 21 miles long. I was in first gear at 15 to 22 MPH, squirrels ran along side so we could feed them peanuts...just kidding.

The boiling point of brake fluid is more than 400f. The issue is brake fluid absorbs moisture, lowering the boiling point. I wouldn't reroute brake lines (the engineers know what they're doing when they design these things) but I would change the brake fluid every 3 years.