brake help

ok back story on my 1984 NA 300zx. back in nov-ish I found it sitting on the side of the road with a for sale sign in the window. it ran just fine, no issues that I could see or notice from the 10 minute drive I took in it. motor is strong, brakes worked. needs some basic love but nothing that was really that bad. talked some more with the owner and he told me that it was his weekend driver for its first 10 years of life (original owner), after those ten years it was placed in storage and just sat there without a weekly start or weekly drive. so of course some things went and now parts need lots of work. but I bought the car cause it seemed like a fair deal for a vehicle that i could jump into and take for a drive. the first thing I did was take it to get new tires, old ones was dry rotted. while there I had them once over the entire car. they told me that the calipers were sticking and needed to be rebuilt/replaced. I told them thank you and that I would do it myself. fast forward to mid jan, (i was driving about 2-3 times a week for a total of 60 miles a week up to this point) I noticed that my brake pedal was getting soft and the travel was getting longer. well because I had already been told that my calipers were sticking and needed to be rebuilt/replaced I had already bought new calipers, new pads and rotors. was just waiting for a weekend that that I had time to do it. load and behold last Thursday my brakes went completely, full travel to the floor with zero stoppage. so I slowly made it home using my e brake and down shifting to get her parked. Saturday I started the process of replacing all the brakes. no major issues except that I noticed that my fluid level was below the min mark on my reservoir, I filled it up and proceeded to do a single person bleed on all 4 calipers. after i had finished it, the pedal was still soft. so i had to wait a few days because of the weather to redo the bleeding with a second person there to help. and this is where it gets weird. rear lines still had some air, I bleed them properly until nothing but fluid came out. brake pedal was hard, and the travel was very short compared to when the brakes went out. moved to the front calipers. started bleeding the lines, 45 minutes later the passenger side was still releasing large air bubbles and the fluid looked like it was saturated with tiny air bubbles. mind you I was constantly checking the fluid level making sure to top it off. i was thinking that maybe the driver side was filled with air and it was pulling the air from there I started bleeding the driver side. 30 minutes later I was getting the same result. this was when i quit for the night and went to top off the reservoir and noticed that the fluid inside the reservoir was filled with bubbles, i cant see any leaks, so i am wondering if anyone else has seen this, or dealt with this or knows or thinks they may know what the hell is going on. my thought is obviously that air is getting in to the system, but i don't know where from, or even how to find it. and yes i tried Google. i cant seem to find anything that talks about how air can enter the system. i know that this post is rather long and probably filled with a lot of useless information but i wanted to make sure that i didn't leave out any thing that may be important.

thanks for the help
mike