This is going to sound crazy but I grew up with a father who is a mechanic so I know this works on a lot of grease stains...
Spray that puppy with WD-40. Not kidding. Just don't run the pants through the dryer after the first wash. Since the WD-40 would have washed out, it's not really a big deal if you forget; just one of my fears that a little of the WD-40 might be left. (Since it's petroleum based,it is also flammable.)
I can't remember for sure if I've used it on brake fluid stains but I know I've used it on various other grease stains.