False signals from that little float switch is not all that unusual,I have had it on occasion on my 87 and my son has had it frequently on his 93.He was going to replace the float switch but recently it has been working OK or at least not giving false warnings.
Pressure in the system when the engine is running is normal from the heat/expansion in the coolant,thats why they put on a cap that is designed to hold a certain amount of pressure,for one reason to raise the boiling point of the coolant and also to prevent the build up of steam pockets in areas of the cylinder head. As for holding pressure overnight i dont understand how that could happen as the cooling of the engine would definitly decrease the pressure from what it was at operating temperature and how high the pressure is at normal operating temp is limited by the design of the cap.The cap even has a built in check valve so that if the cooling system gets cold enough and goes below atmospherec pressure it can allow air to enter to keep from sucking the hoses flat.It seems to me that an engine with a cracked head should loose pressure faster than a good one,but thats just me. Don

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Red Green "This is only temporary,Unless it works!"

97 E300D 157000 miles
87 300TD ?141k? miles