Air locks in coolant; leaking pistons? Test kits

[adebutter]

Two questions around discovery and then action… after local garage bleed;

Is my air in coolant and resultant recent airlock from a leaking pot pushing exhaust in to the water system? Bought a header tank test kit; and thoughts on how accurate or applicable this is to a DG1.9?
Other places are can be drawn in to system?

[cobblers]

If pressure remains in the system with the engine stone cold then the only source can be from a failed piston seal.

[davidoft1]

If pressure remains in the system with the engine stone cold then the only source can be from a failed piston seal.

Or a faulty cap

[davidoft1]

If it’s the change colour type of head gasket tester they are 100% accurate when used correctly