I am trying to eliminate a possible head gasket issue so can someone explain the header tank overflow pipe system. I understand the topping up procedure and understand that the header tank has double seals. the lower seal with the spring closes the cooling system. If this lifts coolant can't get passed the cap and thus runs down the overflow pipe and into the tank near the passenger headlight. Classic and newage are similar. What I don't understand is that this overflow tank can be topped up as min. and max. marked but how can get back into the system. I think my system is over pressurising due to a leak from piston to coolant waterway. Engine is a modded 2.5 with RCM mls gaskets

 

Wouldnt getting a sniff test done be a better route?
Test for the gases that may have escaped from the combustion chamber into the coolant as suspected.

 

The correct cap for the header tank is a double valved one that can suck coolant back into the header tank as the system cools. (winged version)
The expansion tank or overflow tank level will rise and fall depending upon the temp/pressure of the system.

If the rad also has a cap it should be a single valved version (expansion only- no wings) so that any coolant during cooling down only enters via the header tank and the rad is always full.
It should also be the cap with the highest pressure.









Also there are no valves in the heater full flow bypass circuit. Cabin temp control is done by mechanical or electrically operated flaps to mix hot/cold air.

 

Thanks for that, it is a winged cap, I'll take a closer look tomorrow.