Your rad. cap is set to allow air to escape @ 15psi or so, and it has
air contact.  If the hose you use does not leak air around the fittings,
its initial expansion won't count for much inaccuracy in the gauge
reading.  What you are looking for are relative readings: climbing,
running on the flats, rad. on point, stop and go.  Once you see the
gauge out of bounds its time to have a look see.  If the pressure falls
you likely have leakage if it rises extraordinarily try that new rad.
cap you carry.

-Dana-

> This is certainly easy enough sounding. I wonder, however, if the pressures
> you are observing are accurate or not. I say this because air is more
> easily compressed than liquids and there are 2 different states of matter
> here. Any chemists or physicists care to comment?
> Later,
>
> Dave C.
> Williamsburg, VA
>            zzzzzzzzzz__
>          |E[__] [__]|[_]\\
>          | _DC&SP | _ ||
> * * * =-( )---------( )--=
> '85 GL Westy (His)
> '91 Cabriolet (Hers)