A clack valve is a generic term for a valve that only lets water flow one direction. It could be as simple as a hinged flap but is more typically a stainless steel ball which is pushed against a tube under pressure from one direction sealing the tube shut, but allows water to pass from the other.
A Goodall valve is a special implementation of a clack valve that allows you to refill a boiler under pressure using a simple hand pump. A flexible tube from the pump push-fits into a sleeve above the valve and the pump trigger is squeezed creating pressure in the tube allowing water to pass through a clack valve into the boiler.
The Goodall is a rod partially drilled end to end with a hole near the lower end. It is covered with a elastic tube. the upper portion is threaded to fit the boiler fitting and the top is set up for a pressure spray bottle.
The name (GOODALL), is from the person that made the first ones available, Derek Goodall. It is actually a "boiler water injection valve". "VALVE" according to Mr. Webster is [valv] n ; a lid that opens in one direction and not in the other for controlling the flow of liquid or gas in a pipe. The hole in the top being the pipe & the silicone sleeve at the bottom being the lid that opens in one direction.
Although the term"clack" is nowadays used to describe any sort of non return valve, its origins are in the 18th Century tin and lead mines of Cornwall (England.) Most mines tended to flood and enormous beam engines were erected to pump them out. The pump "rod" was actually wood bound with steel straps and typically had a movement of about 6 feet. At the bottom of the rod was a piston, which was little more than a planked platform which fitted the bottom of the shaft. It was a fitted with a trapdoor which lifted as the rod descended, to allow water through, and which closed with a loud "CLACK" as the rod lifted, carrying water with it.
Thanks for the vivid description. I can almost hear those mine pumps operating now.
For some reason, they brought to mind the Water Piston (Humphrey) Pump. These were internal combustion engines which used water for the moving piston. At first blush it sounds impossible, but they were used in water systems in the UK and Australia. Check out http://www.humphreypump.co.uk/operating%20cycle.htm or Google 'Humphrey pump' for more.