When the cooling system is drained and refilled (as would normally be done after it is back flushed) an air pocket is usuallly left in the engine's water jackets. This air pocket will prevent the engine and radiator from being completely refilled with coolant; even though the radiator looks like it is filled to overflowing, and the cap is securely installed. What happens is that, the first time the engine warms up enough for the thermostat to open, the trapped air is moved from the engine into the radiator, where it causes the coolant level in the radiator to drop substantially. The air pocket defeats the siphon action between the radiator and the reservoir bottle; so although the coolant level in the radiator drops, the reservoir bottle will remain full.
That is why it is necessary to recheck and top up the radiator at least twice, after the cooling system is drained and refilled. After that, the air will all have come out of the system, and the reservoir should again work in harmony with the radiator. Some Japanese manufacturers have recognized and addressed this problem by adding a bleed nipple on the thermostat housing (which is the highest point of the cooling system in the engine). This nipple is intended to be left open while the cooling system is being refilled. When coolant begins to flow out of the nipple, it means that all the trapped air has been expelled. At that point, the nipple can be closed, and the cooling system will then have no air in it.
If the air has all been removed from the cooling system, and the coolant level in the radiator continues to drop, while the reservoir remains full; it means that combustion gases from the engine are leaking into the radiator. This typically comes from a leak in a head gasket, or a warped or cracked cylinder head. Less common, but still possible, is a cracked engine block. Such a crack can sometimes be small enough that it does not show up during a cooling system pressure test; but the extremely high cylinder pressure during combustion can force gases through the crack.
There are four ways to test for such a leak:
1> Install a pressure tester in the radiator neck WHEN THE ENGINE IS COLD, and pump the pressure up to just 1 or 2 psi. Start the engine, and watch the pressure gauge. If there is no combustion gas leak, the pressure should not increase further until the coolant temperature comes up (which normally takes at least 4 or 5 minutes). But if there is a combustion gas leak, the pressure will begin increasing almost immediately. Do not allow the pressure to increase beyond 13 psi; or the radiator can be damaged.
2> Remove the radiator cap when the engine is cool, and sniff the air at the radiator filler neck; if there is a small of exhaust gas from the radiator, it indicates that combustion gases are present in the radiator.
3> There is a commercailly available tool to test radiators for combustion gases. It consists of a glass tube with a rubber bulb on one end; which looks something like a battery hydrometer. The user draws air from the radiator up into the tube by squeezing the bulb. There is a chemical in the tube which changes color in the presence of combustion gases.
4> An infra-red exhaust emission analyzer can be used to detect combustion gases in the radiator, by running the analyzer while the sampling hose is held next to the open radiator filler neck. If the hydrocarbon levels increase, then there are combustion gases in the radiator. CAUTION; infra-red anayzers are so sensitive that they will continue to detect hydrocarbons in the radiator for a long time after a combustion leak has been repaired. So, do not use an infra-red analyzer to test an engine for combustion leaks; after a previously known leak has been repaired.
I hope this helps!!!