Taps were invented well before 79 AD, since they adorned houses in Pompeii; more than 40 have been found there. Their design was very similar to recent British taps, but their function was more like a modern stopcock; they were meant to be left on by default (the whole Roman system works on the principle of continuous flow), and would have been quite difficult to turn on and off - too hard to do with the fingers. They appear not to have been fitted as standard with a handle, but a loop, through which a metal bar could be inserted when there was a need to turn the tap.
Taps of the same design, but often larger, were also fitted to some distribution pipes, where they functioned like a stopcock. The largest known seems to be one with a 30 cm diameter on the water main running along the Decumanus (main street) at Ostia.
Water pipe, spout and tap
Lead and bronze
Probably a house with running water e.g. House of Pansa, Pompeii
Original iron and bronze fittings and wood reconstruction
House of Julius Polybius, Pompeii
Only the metal fittings survived, but from other items found nearby, we know that this was a chest for storing medications and medical instruments.
Ancient or Modern Design? http://www.valvemagazine.com/web-only/categories/manufacturing/4947-ancient-roman-valves.html
The author points out a large roman valve in the Naples National Archaeological Museum
Valve Plug Insert with Oval Opening to Allow Passage of Water
This universal use of valves and piping required a level of standardization that is familiar to modern engineers.
The design of ancient Roman water valves is remarkably similar to our modern plug valve design.
What is remarkable is that these 2000-year-old valves are quite similar to water valves that can be purchased at local hardware stores today!
Based on metallurgical studies, the materials used for the casting of Roman valves were:
Several ancient valves that have been tested have results that are very consistent with respect to the materials and percentages used in their manufacture. The Romans had a standard metal alloy mix for valves and they were manufactured with a high standard of quality control. The metal composition shown above for the ancient valves corresponds very closely to the modern ASTM B67 standard for journal bearings used in automobiles and railroad cars.