As others have suggested you need to do a full brake fluid flush…I use my pressure bleeder…after I suck out all the fluid in the resevoir…I hook up my bleeder…it PUSHES…new brake fluid down the res…and out the bleeders on the calipers…thus replacing all the fluid.
SOunds like you need to bleed ALL the brakes and change fluid…IF THIS DOES NOT SOLVE IT…It is your master cylinder…and may be all along… the fluid swap and bleeding should be done as a good precaution and test…it surely wont hurt…like I said if after it does not firm up your pedal…then its master cylinder time… OR as CIRCUITSMITH mentions…the rubber lines can also be a source of a spongy pedal for CERTAIN…
I would do this IN ORDER…
Flush/replace fluid…bleed all brakes…What do you get?
Still spongy?..you can look at your rubber lines when an assistant is pushing on the brakes…Notice any of them inflating like a Balloon? Replace any that BLOW UP…
After the rubber line inspection…if none are blowing up…THEN its Master Cyl time…not a hard job but also requires bleeding of all the calipers when finished…and DO NOT forget to properly PRIME the new master if you get to that point…its called “Bench Bleeding” The instructions are in the box with the new master cyl
Blackbird