I know I'm posting this in two forums... please don't flame

Problem car: 94 Integra GS-R

Anyways, here's the deal. Recently, over the past couple days, my brakes have started to act up. Usually if I haven't been driving too long, the brakes feel excellent and just like they should. After driving for about 30-45 minutes, it starts to feel intermittently spongy(spongey?) with uncertain engagement points (The brakes do not feel like they're consistantly engaging at the same point in the pedal depress).

Thinking it was in need of a bleeding, I did that this morning. The fluid looked rather old, or at least in comparison to the fluid I refilled the resevoir with (The old fluid had a color of slightly used motor oil... Motor oil with like 1K miles on it?)

I am unaware of the last time the fluid was flushed, as I've only owned the car for 10K miles. Also, no ABS codes were being thrown, and the ABS pressurizer was acting relatively normal... well, maybe slightly more than usual (mine usually goes off every other morning).

So, any guestimate verdicts? I'm thinking about flushing the fluid, but I think I'll want to take it to a shop for that. Also, I'm fearing it might be a master cylindar failure, because while I have no specific ideas of what the symptoms would be, it would be expensive, and that's my luck

Also, there's no visible leaks anywhere on the brake lines, and no trace of fluid loss.

Thanks!