Hi from the uk assume you mean the brake warning light on dash which also comes on when h/brake applied ?? your brake fluid reservoir is situated on passenger side under hood/bonnet and is tucked just under the bulkhead below windscreen ? it is awkward for topping up but if you have a small funnel? and a short length of flex hose fitted to the funnel it is ideal for topping up reservoir via funnel with hose attached if reservoir is very low then you may have leakage from either pipe/s or wheel cyl's? this reservoir is also for the clutch hydraulics check the slave cyl for leakage too ? other than this ?? the drop in fluid level may simply be due to brake pads/shoes wear and as these wear low the fluid is displaced from reservoir to cyl's to compensate as pistons of cyl's move out taking up wear of pads/shoes?hope this explains ok ? may i suggest you check your pads/shoes for wear ? if you have not done so recently ? just for your own safety as well as others on the road best wishes