Have a 91 Cavalier 2.2 auto non-ABS. It only loses brake fluid when its cold. Replaced the drivers front brake line from master cylinder to caliper as rubber piece was cracked and rusted. Can't find any leaks anywhere, had a slow loss of fluid last fall. We jacked the car up and put cardboard under all 4 wheels and pumped the brakes and couldn't find any fluid loss. As soon as it got cold out no more fluid loss. Now since it warmed up losing fluid again. I check it everyday and its like full then all of a sudden empty.

Im thinking master cylinder maybe? My dad said something about the brake booster filling up with fluid? How can you tell if the master cylinder or brake booster are failing? Im unemployed and this is my DD, would used parts be a bad idea? The master cylinder is like $50 but the brake booster is $120 so I don't wanna buy any unnecessary parts. Or would it be best to take it somewhere for a brake inspection and then replace whatever they say is bad? I know a lot of questions thanks in advance.

Unbolt the master cylinder from the booster. It should be bone dry between them. If there is seepage, it's time for a new master. Either way, as long as the brakes still function normally, I doubt it's the booster.

Ok thats what the culprit seems to be I don't drive it very often but I might have to make it a dd again so deffinitly have to figure this out