You shouldn't have to add any antifreeze at all. There shouldn't be any
bubbles either.  Something is wrong. Pull the oil dipstick & see if there's
any antifreeze on it.  If so you have a cracked block, warped head, or blown
head gasket. Any of them are big dollar repairs.

I would start with a basic cooling system pressure check. If you have
a leak, or loss of coolant externally, there could be air pushed back
into the reservoir when the pump spins up. The bubbles can be caused
by the air purging through the system, the reservoir(surge tank) is
often higher in the cooling system layout. A loss of fluid due to a
leak would create an air pocket that would not purge until coolant is
circulated. Assuming that the vehicle did not freeze, nor is
overheating, might eliminate an internal leak. A secondary test for
internal leaks is a block test that uses a special fluid, in a special
tester, that pulls air from the closed cooling system through the
fluid. Any presence of hydrocarbons(products of combustion) causes the
fluid to change colors, usually from blue to green or yellow depending
on the severity. This testers are available at NAPA, and your major
tool dealers(snap on, MAC, MATCO), i'm not sure who else has them. The
block tester is pretty reliable at detecting internal leaks. There are
circumstances where an internal leak can exist and not be detected,
like a cracked intake manifold, leaking intake gasket, etc...,
situations where combustion pressure is not present, or higher than
cooling system pressure.

Monthly fluid loss in normal driving from a properly sorted out
cooling system in good shape should be negligible, so no doubt
something's going on.   I smell a head gasket (and/or worse), but
there are several cheaper and easier things to look at.

I usually  don't recommend "shotgun" replacement of parts (as opposed
to diagnosis), but a radiator cap is trivially cheap and easy, and the
thing has a lifetime of a few to several years so you probably need
one anyway.  (Amazing how much cooler an engine with an aging radiator
cap starts to run when a good one is installed, even absent other
physical problems with a cooling system.  It's meant to run with a
certain amount of pressure, and the radiator cap is one of the items
tasked with holding that pressure.)

Nose around all the hoses, also, to be sure it isn't leaking out.  And
have a good look with a bright flashlight at the vicinity of the water
pump when the engine is cold and again when hot (hands safely out of
the way, of course).  Soft-parts overhaul of a cooling system is not
that difficult for the do-it -yourselfer nor very expensive, though
perhaps where you live it isn't the season.

As others have advised, a pressure test is a good thing.  I'm told
that a smog tester can also be used to sniff for exhaust gases around
the radiator cap.

Hoping it's something easy and cheap,
--Joe

Thanks for the information.

Dennis
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