Are you using wire braided brake lines for your fexible spots? If not that will cause the spongy feeling you are experiencing because the rubber lines will expand from the pressure.
Brake fluid absorbs water from the atmosphere and over time it gunks things up. That is why it is recommend to flush the system at least every two years, more often in high humidly areas.
If you drain all of your brake fluid and switch to DOT5 100% Silicone fluid you will eliminate this problem. Silicone and water do not mix and does not attract moisture. This is a PITA though beacuse DOT5 cannot be mixed with DOT3 or DOT4. You must drain and flush the system completely. The US Military uses DOT5 in all of their vehicles.
