A couple of months ago I had a brake line bust on my 03 monte carlo 3.4 liter..Had to replace the whole rear brake line up to master cylinder.Brakes were all bled and when I drove it the brakes felt softer than normal. a couple of days later as I was driving the pedal went to the floor and the brake light and low fluid light came on so and the master cylinder was almost empty.i filled the master cylinder back up up and went on my way checking the master cylinder once a week and it looked good...I was driving home yesterday and stopped to get new tires put on and the brakes worked as normal(soft pedal).When I got back in the car to leave the brake pedal went to the floor I babied the car all the way home and when I got in the driveway I pumped the brakes a couple of times and the brake light and low fluid light came on again(hadn't checked fluid in a couple of weeks).I them checked the fluid and it was low but not empty..No leaks anywhere And wondering what my problem could be. maybe the master cylinder and booster? Thank for your time

 

If your loosing fluid and having to top off one of two things come to mind:
1. You have a leak somewhere and don't see it yet. I recommend checking all fittings and unions on your new brake line (I've had times that my fittings where not tight enough or I had a bad flare in a line and did not get a good seal). It will take having one person watch under the car and another pumping the brake.
2. You have air in the line or the caliper and it's escaping somewhere causing it to look like you are loosing fluid.

I'm more inclined to go with option #1.
If you are loosing fluid, it has to come out somewhere. I had a caliper with a bad seal on a piston leak fluid out the piston (that is rather rate in my opinion).