Brake fluid leak

Can't find any evidence of the leak and have been told it may be the master cylinder, which doesn't look wet. How to diagnose a master cylinder leak??? Also the brake pedal seems to go down [almost to the floor] when I start the engine from a good pedal before starting.
Thanks for your help.

Pedal going to the floor with the help of the vacuum assist is not a good thing. As far as leaks go, also take a good light and examine the clutch master cylinder. From the inside of course... you may have to remove a cover. Buuuuuuut, the brake pedal going too far down does not point to anything hooked up to the clutch.
cheers,
Douglas

Does your fluid level need topping off regularly? A brake fluid leak can be anywhere there is a brake line or wheel cylinder, in addition to the master cylinder. Are there any wet spots anywhere underneath your car? Are there any wet spots down the inside of any of your tires after the car sits overnight?

Syracuse is known for its lovely rust, and your car is almost 20 years old, brake line will rust and leak in odd places.

If you aren't loosing fluid you don't have a leak. If you are loosing fluid but don't see it anywhere it's going into the booster.

If the pedal goes to the floor and you don't have a leak your master is bad. The fluid is bypassing the piston seals.