A compression test is a measurement of the momentary peak pressure that comes to exist during the compression stroke of any given cylinder. It is simply an absolute measurment of how high the pressure gets, it does not indicate much else.

The leakdown test is a procedure that is used to determine how well/long the conditions, i.e. valves, rings, cylinder walls, pistons,head of a given cylinder can maintain a given pressure. A known pressure is applied to a cylinder at TDC and to loss of that pressure it measured over time - The faster the applied pressure is lost the worse the condition of the cylinder! Some diagnosis of potential cylinder component issues is possible when you listen for or observe the leakage of air pressure from the cylinder.

Leaking air heard at;

Intake manifold = intake valves

Exaust manifold/tail pipe = exhaust valves

Valve cover oil filler hole = valves, valveseats, valve stem guides, or valve stem seals

Oil dip stick = worn piston rings, cylinder walls

Coolant resivior (bubbling or hissing)= head gasket, cracked cylinder liner

Leakage at the valve cover alone usually indicates top end /valve problems.

Leakage at the valve cover and dip stick usually means piston ring/cylinder wall problems.

Head gasket/cylinder liner/cracked head problems can leak air pressure into either or both the oil circuit or coolant circuit.

Both tests used together is the best way to go, a compression test will determine if the minimum threshold conditions are met, i.e. is there enough compression meeting the spec for each and between all cylinders? And a leakdown (loss of pressure over time) test of the cylinders will determine how well the compression values are maintained by the components of each cylinder.

If you have a diesel compression tester you can use the injector/glowplug adapter, a check valve, open/close ball valve, and a couple of gages and a tire inflator fitting to make your own leakdown tester.

assemble the parts in this order:

Tire inflator fitting - T - ball valve - check valve - T - adapter to the cylinder, at each T attatch a presure gage (use gages that have a full scale twice the pressure you will be using for better accuracy).

To use the compression tester adapter you only need to remove the tire stem valve from the adapter that is usually functions as a check valve allowing pressure from the cylinder to the gage because in the leakdown test you will be appling pressure from the gage array to the cylinder.

Attach the adapter to the TDC cylinder - open the ball valve - apply shop air to the tester - when both gages read the same close the ball valve (this will hold the first gage at the applied pressure reading) - observe the rate at which the second gage drops over time (this second gage reads the pressure in the cylinder)

P. S. It may be wise to secure the crank in position to prevent engine rotation when air pressure is applied to a cylinder, in theory the applied pressure could force the engine to turn. Of course the higher the pressure the more likely that this could happen.

On another note, one could assume that using the highest level of air pressure up to the compression pressure measurement level would be safe and would give the most realistic/accurate measure of the engine condition.