It is more of a general "manual transmission+hydraulic brake" issue than a del sol issue, but I may need some guidance.

Last week I changed my brake pads because the old ones were starting to squeal like a greased up hog, everything went fine and I drove it around no problem. Fast forward to this week and the brakes are fading at stoplights. I have checked the brake fluid multiple times and the level never goes down, so my guess is that there is air in the lines.

Does this seem reasonable?

(I just really wouldn't want it to be the master cylinder going out, I'm trying to sell the car soon)

 

That would be the best place to start, with 2 people you can do the 4 corners in like 15 min. And also I'd check your soft brake lines for holes