I think the purpose of the particular jiggle valve nearest the water pump (not the one in the thermostat itself) is to vent air. Some will argue that the ball should be removed to stop it blocking up.
In the earlier system (pre PRT) I suspect the other jiggle valve in the “up and over” hose is to stop it being air locked.
But it would vent any air too but with the ball sealing the vent it would prevent circulation.

So, if it is not fitted in the 4-way system there must be an unrestricted flow to the expansion tank. Exactly how vigorous will no doubt depend on the fluid dynamics. 
What I am seeking is someone with a PRT fitted as standard and to find if there is a jiggle valve in that small bore spur from the 4-way T to the expansion tank, and if there isn’t is there a continuous flow when the system has reached full operating temperature.
Anyone help please?

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