when i have done my brakes, they were spongy.
tried bleeding to the steps in nissans manual.
felt the same.
tried bleeding another 3 times.
felt the same.
took it for a drive, and stopped just ok.
next day, brakes felt firm, havnt had the problem, until next time i touched the pads.
same thing, turned the car of for an hour, and it came up ok.
WTF???
anyway, personal experience i supose.
o/p, hope youre enjoying the new brakes.