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I may be reading that wrong, but normally the pressure cap would NOT be on the expansion tank, but on the engine or water heater tank. The higher one I would think. Expansion tanks usually have just a silly splash prevention cap.
This is the best explanation I can find.

http://homepages.donobi.net/sufuelpumps/Other_Subjects/Cooling_and_radiator/EXPANSION_TANKS_COOLANT_RECOVERY_SYSTEMS_AND.pdf

They use the terms header / expansion tank, and coolant recovery tank.

Again, though, they agree that the former is pressurised - although on the interweb you see the term used for both.
 

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Oh sheet! Just when I thought I knew everything...
 

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This is why industrial fluid systems have flow labels on the pipes and more labels on the valves and tanks. If you've got a PTouch around...makes a good excuse to inspect the whole system and make sure there are no cracks and leaks or hoses that are drying out.
 

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It could mean that you have a head gasket leak. However, to test this you need to install a cooling system pressure tester an monitor the system pressure while running the engine.

There has to be room for expansion with temperature changes. Either maintained air space in the system or a pressure relief valve in the rad cap with an overflow (expansion) tank.
 

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There is a great diagram and explanation of the cooling system on a diesel here;
speed

This inspired me to implement the Thermostat Bypass Connection to the water heater on My O'day 35 with a Universal M25.

Buried therein is this diagram;
 

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It could mean that you have a head gasket leak. However, to test this you need to install a cooling system pressure tester an monitor the system pressure while running the engine
An alternative way to check for a blown head gasket, cracked engine block/head, or pin hole in the exhaust manifold 'water cooling' circuit is to use a fluorescing 'black light' indicator dye. When there is carbon monoxide present (due to the leak) the cooling water will 'fluoresce' (glow) when 'black light' and the indicator dye is used.

Manually pressurizing the cooling circuit to 14-15psi may not generate sufficient force to cause such a leak to 'flow' (without normal alternating pressure/vacuum in the combustion chamber(s), .... especially for cracks and head gaskets problems; although, if the leak is severe enough such 'may' show the leak into the combustion chamber(s) with the injectors removed - you SEE the water. For exhaust header leaks (probably #2 cause), you can easily remove the exhaust manifold and 'pressure test' it - perform a 'pressure hold test' to about 60 psi on the 'water side' of the manifold - SEE the water entering the 'gas side'.

Loss of cooling water without seeing water usually is due to 1. blown head gasket, 2. cracked block/head, 3. pin hole in the exhaust manifold.

Ed. - The reason why Im listing this is a very small combustion leak into the cooling circuit will continually pressurize the cooling circuit plus will add 'gases' which will accumulate under all the upper surfaces of the cooling system.

;-)
 

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We used to use just the test fluid (1/4 of the kit price) added to the coolant. Turned the coolant blue if there were exhaust gasses in it, and a turkey baster works just as well for sucking up samples from the radiator cap.
 
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