Disc brakes feel spongy no matter how much I bleed them
What fluid? DOT 5 will do that.
Uh, NO. Harley never had that problem and ran DOT5 for three decades. Properly-bled DOT5 will NOT do that.
OK OP, time to do yer homework.
Install master on car and bench bleed. Do not hook up lines. Instead, cut some pop can metal and place under the lines, and then screw the lines down, so no fluid can flow. I think you can get dead-head caps for this, too. Pump the brakes. Firm? If yes, keep looking (down to the next paragraph). Not firm, even after a bench bleedin'? Replace master.
Hook up the front brakes only and bleed. (Make sure you're hooking up the front
system, with the disc brakes, and not necessarily the front line on the master cylinder). Pedal firm? If so, it's a rear brake problem. If mushy, it's another issue. You can keep going down the line.
It might be a hose going bad.
If the fronts keep a firm pedal, the rear brakes might be misadjusted, etc. but at least you've narrowed it down to a system. You can even narrow it down to a corner this way, if you want.
