As we all know the later design of the thermostat is the "PRT".
Prior to this the thermostat was located at the cylinder head. With the early system it included two "jiggle valves" (one at the cylinder head and one in the up-&-over hose close to the other end of the cylinder head where one hose join the water rail).

The PRT system retained the jiggle valve in the cylinder head, and we can debate the usefulness of that, but the other appears to be omitted. As the jiggle valve is intended to dispel air but seal against water flow that means that there must be a continuous flow of water to the expansion tank with the PRT arrangement via the 4-way connector.
Is this correct? If not where is the other jiggle valve please? 
If no second jiggle valve should there be another restrictor in addition to the one that fits against the water rail to improve the flow to the heater? If so what size and where does it fit?

Read More...