We have a nice but old Cannondale RT 1000. Since I have a bad back, I was thinking of getting a new Brooks with springs on it. (Capt. position.) Which one of the Brooks line up is best for bad backs?

If you have the funds, check out the Selle Anatomica site as they are having a 20% off sale with their saddles and you can get the Kinect seatpost as well. It may not work for captain position as the spring section of the seatpost is about 3 or 4 inches.

The thing I hear about the Brooks is that you have to break them in and it takes a while. Haven't tried them myself but the Selle Anatomica saddles are great. I have an X1 that I'm a bit overweight for and have since added an H2. The leather is great. Done several 70+ mile rides on them. I'm one of those guys who doesn't ride at all for 5 months then decides to do a century. Not sure I could do that if I had a hard leather saddle that wasn't broken in.

The Selle saddles don't have a spring option though.

Brooks B-67 is my go to. The sprung models don't really have a break in period. They just work. That being said, it's just my opinion...

I’ll second the recommendation for the Sella Anatomical saddles. After trying multiple saddles, this was finally the one that worked for me. It took about 100 miles to “break in”, vs many more miles for my friend’s Brooks saddle to break in (but both are now perfect for each of us). I now have over 22,000 miles on the saddle (mostly cycle-touring where I’m in the saddle for long hours). And with the periodic sale prices on Selle Anatomica, they’re a decent bargain.

I have a B-67 with about a million miles...now on it's third bike, my touring bike. I have a Brooks Flyer (B-17 w/ springs) on my Paramount. B-67 is better for a more upright position, a bit wider and a little shorter.
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You're asking strangers on the internet to diagnose and treat your back problem?
There are many different types of back problems with different treatment regimens. Finding the best position on the bike is more likely to provide you relief than any saddle.
I think you're better off visiting a sports medicine doctor.
Brent

Nope. He asked for a saddle recommendation.

"Which one of the Brooks line up is best for bad backs?"
Brent