A very effective method for checking for cooling system leaks
is to pressurize the cooling system BEFORE you start the engine.
Once you start the engine you can be fooled by the system
pressure as the coolant expands as it heats up. You could be
losing coolant at such a low rate that the pressure remains normal for
quite some time. The leak could be internal within the engine through
a water O-ring failure.

Pressurizing the system when its cold on the other hand will immediately 
show you if there are any leaks.  Here is the guts of the little 12 volt 
electric compressor Perry and I used to trouble shoot his cooling system failure.
Available in any auto parts store as an emergency tire pump.

I am going to mount one of these permanently under the cowl (without 
the black plastic case) with a switch on the instrument panel. It only 
takes a few seconds of operation to get the system up to 30 psi.
 
Paul Lamar
 
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