How reliable are the Coolant Bottle Pressure Caps on ZT/75 1.8t engines?

For the first time in over two years ownership, my 1.8t ZT left a patch of coolant underneath the Expansion Bottle Area. Has been very warm the past few days. That wet patch following an 80 mile round trip. After a good few hours, I removed the the cap and topped up the coolant loss. For good measure, I used another Expansion Bottle Cap which looked identical but, instead of the original show "200" on the underside of the Cap, the replacement showed "140". No idea if that means anything. Otherwise the two looked identical.

In yesterday's heat ~ the temperature showing 26 C on the Instruments took the car for a quick five mile run up onto the Cotswolds. Parked up and lifted the bonnet. A very hot engine but no sign of any coolant escaping.

Carried on driving including some brisk sessions of acceleration using full boost. Car went very well. Did about another twenty miles in the continuing heat of the day. It was very warm.

Parked up back home and lifted the bonnet. Not surprisingly, the engine was very hot and the top hose was very firm but still has some give in it when squeezed. About the same as my T-Series 620ti Turbos on a hot day driven that way.

This morning, I looked under the car and could not see any tell tale wet patch, even a drip or two. Nothing. 50-50 OAT Antifreeze does not dry up and disappear quickly as plain water would do. So looks like no coolant past the replacement cap. I removed the cap and checked the level. The top of the Bottle where the MAX index and tubes are was very wet, but not under water.

So, have I got something to worry about or, given the very warm weather this past week or so, all things considered it may simply have been a worn Pressure Cap.

Even so, will watch and monitor closely as that may have been a premature first indication that all is not well. At no time did I hear the Fan being activated but being on the move, the air passing through the Radiator at swift road speeds should provide sufficient cooling.

So the question remains. Do these Caps need to be replaced at regular intervals on a nothing lasts forever basis? The used replacement seems to be doing its job well... so far.

Any advice or suggestions here will be most appreciated.

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How reliable are the Coolant Bottle Pressure Caps on ZT/75 1.8t engines?