Disc brakes, hydraulically powered, do get mushy over time. This can be caused by infiltration of water into the brake line or master cyl reservoir. Water does not compress the same as brake fluid, so it will cause the brakes to be soft or mushy. The repair is to flush out the brake line.
Similarly, if there is nasty gunk in your brake line, which happens on old bikes, your entire hydraulic system should be flushed out and refilled with clean brake fluid.
Also, if you have leaks, fluid will seep out while air gets in, and you will get the same problem. Look around for any damp spots, and tighten or replace the connections as needed.
You could just be low on brake fluid. If your master cylinder reservoir runs dry, you will have to refill it and then bleed the brakes. If it just gets low, you can fill it up.
You could also have a bad master cylinder, which doesn't push the fluid like it used to. The repair here is to rebuild or replace. Many Japanese motorcycles can actually interchange their master cylinders, even across manufacturers, so if you have a donor bike from another manufacturer, give it a shot.
Finally, you might just have to bleed the brakes. The brakes will need to be bled any time that you open up the hydraulic system to the elements, with the exception of the top of the master cyl reservoir, you don't need to bleed after that.