I have to refill my brake fluid reservoir a lot. Probably once a week

Brake master leaking into brake booster or rear wheel cylinders leaking and as u drive it slings brake fluid around inside of wheel and u never see a leak.

Fill the master cylinder to proper level , then sit there and pump the brakes firmly , holding pressure for a minute or more at a time , depending on how fast the pedal sinks. AFTER at least 10 minutes of this , get out and look under car for fluid dripping/running down a wheel or from a rusted line. Then if nothing seen , try to start it and be able to watch the exhaust for some smoke , if fluid is bieng drawn into the booster it may cause some smoke . Also , look at booster around where master cyl. bolts up and see if any paint blistering.