Cooling system not pressurising

[wayneg]

Having sold the Old p38 the new one is now my daily driver. All seemed well until my 1st longish trip towing a camper. After 165klm I got a couple of flashes on my low coolant alarm so stopped for a look, Coolant was bubbling out under the expansion tank cap. The car was not running hot nor the coolant gushing out. I let things cool off then topped up the level (not much) and carried on, no more warnings. After the weekend away I returned to Perth, nearly home and a couple of flashes, stopped to check and all good, here I noticed there was no pressure in the system.
I now have looked into it more and the system never pressurises. The car has a new rad, new expansion tank, new water pump, new viscous coupling, new cap, new belt and pulleys. I have done an extended pressure test over several days and its 100% leak free.
I now note that if I squeeze the top hose I can hear air escaping from the expansion tank cap. Tried another cap and the same. I can only presume that the expansion tank itself is not right if the cap wont seal, any had a similar experience.
The car runs sweet with no overheating

[bee utey]

If your new expansion tank doesn't hold pressure I'd be carefully inspecting the area that the cap seats on, looking for defects. Badly made plastic items aren't rare, you know.

[wayneg]

The cap has 2 o rings on it which seal against the side of the neck ( I presume )
If the internal size of the neck is to big it obviously wont seal. If the neck inside is tapered and the neck is to long the cap wont go on far enough.. just thoughts,,,,, have already had it suggested I take a couple of mm of the top of the neck.
I will get another genuine tank and cap from the UK. I have no idea where the new tank on the car came from

[Hoges]

How full is the tank? They can develop a split around the outer ridge area where the two halves are plastic welded together...

[wayneg]

The tank is at the correct level when cold and raises about 1cm above the seam when hot. There is no loss of coolant apart from the day of driving at 110kph for 2 hrs and this was only enough to set of my alarm which is about 1cm below the cold level. Normal driving no loss of coolant, just no pressure.
I dont understand how the cap works, 2 o rings and a flat rubber washer to seal, what gives to release over-pressure. What I dont understand is with the cap on tight and coolant at the correct level if I squeeze the top hose the coolant level raises as I would expect, I can clearly hear air escaping through the cap so there can be no seal.

[TheTree]

Hi

I recently replaced my cap and had the same problem.

Bought another AllMakes cap from the UK and problem solved

Steve