I have bled the brakes on my 87 GXL repeatedly and thoroughly.
With the car running, but not in motion, if I press the brake pedal, it will slowly fall to the floor.
If I'm driving, the first time I press the pedal, it will go almost to the floor, then I pump it up, and hold it, it still slowly falls, and If I let off, I have to repeat the process.

I can not find any fluid leaks.

Does this sound like I haven't bled well enough, I have a hidden fluid leak, I have a vacuum leak, or possibly just a bad MC?

Secondly, Rock auto is closing out Master cylinders, But only those for cars with adjustable suspension. I do not have adjustable suspension. Will that master cylinder still work for my application?

 

Bad master cylinder, the internal seals are bypassing fluid, that's why you're getting the sinking pedal.

I had this problem in my 6mo. old master cylinder, it was a reman. Bought a brand new one and haven't had any problems since.

The MC should be the same for cars with and without AAS, as long as the calipers are the same (should be 4 piston)

 

Front are 4 piston, rear are single.
5 lug axles. thanks for your help!

 

AAS master cylinders have a sensor in them (according to mazdatrix's parts catalog), but I'm not sure if it'll work fine for non-AAS cars.

As for other differences, 4 lug and 5 lug n/a cars have the same master cylinder.

Someone correct me if I'm wrong, but I thought 4/5-lug cars differed in the proportioning valve and brake booster.

 

Yea it's bypassing inside .. either that or it is just a poor bleed job..( geod919 a vacuum leak shouldn't produce these symptoms )