A couple ways. Use a turkey baster. Open the bleeders and allow it to gravity bleed.
If the Master Cylinder is defective then replace it.
its the brake resivior
DOT3 Look on the top of the master cylinder cap.
the pedal will slowly depress with no loss of fluid
DOT 3. Listed on the top of the master cylinder cap.
At the master cylinder brake fluid reservoir located driver's side next to the firewall on top of the master cylinder. Use DOT 3 fluid.
Check brake fluid level in master cylinder Make sure parking brake is fully released
In the Master Cylinder mounted on driver's side near firewall.
Normally it means the brake master cylinder is low of fluid. But it can indicate another problem.
The one on top or near the master cylinder. On the cap is written the type brake fluid to use.
My 1992 Honda Accord Brake Lamp came on several times after I had the Master Cylinder replaced. Turns out the fluid had gotten low. Once I refilled the fluid level, the light stopped coming on.
Normally it means the master cylinder is low of brake fluid which also means your brake pads are worn out or you have a leak.