Motorcycle Repair: loss of brake fluid, 70 Electraglide, piston cup, electraglide

Question
Howdy.
Took my bike out of storage, went for the first ride of Spring, and discovered at the first corner that my brake petal went all tyhe way down with no noticable effect. Immediately returned to the garage.  Thought of simply adding brake fluid but it occurs to me that if all the fluid is gone, that indicates a leak, and a visit to the mechanic is needed, not just "topping off".

Any ideas of what might need fixing or replacing?  How involved a job are we talking?  Parts that you recommend to change, replace or upgrade while I'm at it?

Thank you.

Answer
If it's not leaking anywhere it's probably the master cylinder worn out and you can get a rebuild kit with piston, cup, springs, o-rings, etc. and just run a brake hone lightly through the master cylinder.  Make sure the bleed hole(s) in the resevoir are clear.  Use a needle to clear them.