Series 1 brake issues

I took my 1971 Series 1 auto out for a drive the other day, and the brakes felt soft.  The pedal needed to be pressed down all the way to brake.  I pulled the reservoir cap off and the one closest to the firewall had almost no fluid.  Does this suggest it needs a rebuild, or time to replace?  I cannot find leaked fluid anywhere. If a rebuild or replacement is necessary,  can anyone suggest a reasonably priced replacement? Thanks!