I'm pretty sure i've read a similar post before but just to make sure...
After long periods of driving my brakes will gradually lose pedal pressure till eventually they'll sink all the way to the floor with very little braking power. This is extremely un-nerving and has almost caused an accident. Is the the brake master cylinder going out? Is there any other possibilities? I just wanna make before I start trying to fix it.

 

If this is happening with normal driving - yes. Replace!

 

yeah same **** happened to me over the weekend...almost killed me....after spanking a 95 240SX....

 

hah, same here. just bought mine today $95 w/ my 20% off coupon from dealer
it's scary driving wit the brake pedal sinking

 

Check your break lines [bleed them incase air/water/debris got into them]

Check to see if anything is coming out of your calipers.

But, most likely it will be your Mast. Cyl. Just tyring to give you other alternatives.

 

Soft, spongy brake pedal all the time, pump it a couple times and its good? That would be air in the lines. Bleed the brakes, flushing lots of fluid thru each corner.

Usually ok, but soft spongy pedal after a really good workout of the brakes? Brake fluid boiling point has gotten too low. Bleed the brakes, flush lots of fresh fluid thru.

Usually ok, but when you're resting your foot on the pedal (at a stop sign) it slowly sinks to the floor? Doesn't always happen when you're pressing hard on the pedal. That's a bad master cylinder. Replace it. This sounds to me like your description.

You might have more than one of these at the same time...

Of course, if anything is leaking, that's a problem, too.