The hydrovac is still used today in thousands of applications. Most passenger cars use a diaphragm rather than a piston but the mechanics are exactly the same.
The mandate to switch from single to dual circuits was 1968 IIRC by the Government.
As long as you keep the leather and felts oiled it should last many, many years (piston vacs need oil about every 6000 miles). Most rebuilt vacs will use silicon grease which is maintenance free.
