I just installed a new front wheel and Avid BB7 brake on my utility hauler bike, which already had a Nashbar rigid fork with disc tabs on it. I went to my LBS and asked for compressionless brake cable housing, but they didn't have any, so I just got 3 feet of standard spiral brake housing. Now that it's all installed the lever feels pretty spongy, and in fact I can touch the handlebar grip with the brake lever if I pull hard enough.

I've heard that compressionless brake housing is supposed to be great for mechanical disc brakes. Of course I wanted to hurry up and get it set up which is why I just used the regular brake housing in the first place. So will it make a noticeable difference?

Not once everything is settled in.

I cannot imagine how spongy our tandem would be without compressionless brake housings. The brakes have great feel despite the speeds we are hauling down from with our combined weights. I think it's a good investment.

Is the BB7 the road or mtb version, and are your brake levers canti/caliper levers or v brake levers?

Two points about housing.

1- standard brake housing is a fully compressed coil on coil spring. As such it's already pretty compressionless. Sometimes people mishandle it, and make overly sharp bends which cause the spring to relax (move from compressed coil on coil), which will make the system feel spongy.

2- so-called compressionless housing will often make a cable feel firmer, but is prone to two typed of sudden catastrophic failure, which make it unsuitable fro brake use.

a- because the wires are nearly lengthwise, the compression load when the brake is applied causes them to bow outward. This is resisted only by the plastic cover, which has limited strength and can fail, especially if degraded by UV and time. Some housings prevent this by wrapping Kevlar or CF around the wires under the plastic. These are rated for brake use.

b- regardless of whether the housing is buttressed, the lengthwise wires tend to work to the center where the end meets the typical conical bottom ferrule or brake fitting. This can cause some binding, and in extreme cases can extrude through the fitting leading to failure. The issue can be solved with flat bottomed ferrules of sufficient strength to handle the load.

So compressionless housing can be used, if it addresses the two limiting factors. But may not solve the real problem, which I suspect may be sloppy fits at the ends. Take the system apart, and grind or file the housing ends flat so they sit squarely in the fittings. Then do a few hard squeezes to settle everything before adjusting the brake.

Other issues may include non cable flex, such as the brake lever arm or parts within the brake.

I've had problems adjusting the brakes on my cargo bike, front center pull, rear TRP Road. With Dia Compe Blaze (or similar) levers.

I think the solution is to replace my Blaze levers with Tektro RL520 levers.

Unfortunately, the front brake is where you will notice the least difference since the cable goes through less housing than the rear.
Assuming you have the proper levers for the brake, it could be a few different simple things.
You should make sure that your housing ends are cut squarely so that the ferrule is solidly on and not moving on the housing when you pull the lever. Poorly cut housing can leave a 'fang' that acts like a spring pushing the ferrule away. When you grab the lever, the ferrule compresses back before you even begin to move the brake pads and everything will feel mushy.
Another reason could be that the cables we're not of the pre-stretched kind. They will offer a little more give at first until everything gets seated and broken in. Once that occurs, you may need to take up the slack by either a barrel adjuster or loosening the anchor bolt and pulling all the slack out.
However, if you don't have the proper lever for the Avid brake than I would start there and make sure your levers are compatible with that brake. Your levers might not be pulling enough cable.

Regular spiral brake housing is good stuff. Routed properly and cut cleanly it will offer all the firmness needed for any decent brake. You have an actual problem somewhere and you need to figure out what it is. Hint: it's not something that can be fixed with compressionless housing. It could be in poorly cut housing, poorly routed housing, or the result of a lever/brake mismatch.

Other sources of flex when using rim brakes include flexy calipers, flexy frames/forks causing canti studs to shift outward, and soft squishy pads.

On a tandem with an extremely long rear cable, sure. On a front brake? Regular housing should handle that just fine. Something else is going on here.

I was asking specifically about compressionless BRAKE housing, with the Kevlar around the wires. I know not to use compressionless shift housing for brakes.

I had V-brakes on the bike before, and switched the front to the Avid BB7 MTB version, using the same levers. The levers are integrated brake/shift pods, Shimano ST-EF51's.

Since you have V-brake levers it can't be a lever/brake mismatch, so your parts are fine. But the front housing loops are WAY too long, like cartoonishly long. I wouldn't trust that the housing was cut well if the person doing the setup is that bad at getting the length correct. Is this something you can fix yourself? See also: Cables

Shoulda stuck with the V-brakes

I installed all the cables myself, thanks, and they're that long because the bars are FSA Metropolis ("mary jane" style I think?) and need to be that long to not get kinked up as they go around the head tube. When the cables come out of the brake/shifter pods they're angled forward instead of straight across as with normal flat bars.

to overcome the blow out potential of using SIS low-compression Gear housing, bundles of wire on end,
Jagwire added a Braided sheath of Kevlar within the outside covering, to be used on cable disc brakes..

hmmm.... now that I think about it I may have forgotten to put a cable housing end cap on the end that goes into the caliper. I'll check and fix that up this evening.

I'm a big fan of Jagwire Rip Cord housing. I've used it on disc brake applications that were previously less than marginal and personally found that it made a huge difference. Now I won't use anything else. If spiral wound housing has so little give why is it inadequate for indexed derailleur use?

It looks like they dropped the name "Ripcord" and changed it to "Mountain Pro" and "Road Pro" housing.

There is compressionless housing designed for brakes. Two I've used are Yokozuna and Jagwire. The Yokozun cable is wrapped with a thin band of steel to prevent expansion failure and Jagwire is wrapped in Kevlar for the same purpose. The Yokozuna is very stiff and a bit larger diameter than standard. The Jagwire is less so. I enjoy the reduction in lever sponginess these have provided and will continue to use these or similar in the future.

Sounds good. I was looking at the version from Odyssey and I like how they have specially-designed cable end caps to help further protect from cable blowout.

Well I put a cable end cap on the end of the housing going into the caliper, and now it does feel a little better, at least equal to my rear Tektro V-brake.

The vast majority of our tandem uses compressionless housingless. It's called a frame tube with casing stops. Andy.

As someone who has done exactly what you have done, I can tell you from my experience that using compressionless brake housing will make a great difference in the feel and operation of the brake. I used Jagwire, and used the furrels that came with the housing. The spiral housing that I had when I put the disks on was like a sponge. The more I pulled the lever, the more the housing compressed. The rear brake had full housing, it was almost useless. With the compressionless housing I can lock it with ease.