Don't assume the worst case! (blown gasket)

If the reservoir cap is bad, there's no pressure being held on the cooling system, which means it will boil at 100C (water at sea level) or about 120C with 50/50 antifreeze. These temperatures are quite likely after shutting off, when coolant stops circulating around exhaust ports and combustion chamber surfaces. Temporary, localized boiling can then occur at those spots, resulting in bubbles in the reservoir. With the engine shut off - no combustion is occurring, therefore no pressure in the cylinders - so I can't see how a gasket leak could cause what you are seeing.

Test your cap and/or replace with a known good one and see what happens.

If you really suspect a head gasket leak, there are test kits at auto parts shops where you can put a bit of chemical in your reservoir and if it turns some color, there are combustion products in the coolant. THEN you have a gasket leak. Or go to a good radiator shop and they will have a sniffer which can detect combustion gasses in there.


--
Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)

A method I have used is to feed air into the cylinder from either a compression tester fitting or a hollowed out spark plug with an air line fitting brazed to it. The same tool I use for holding valves closed when doing valve seal replacements. When you put air in the cylinder, just watch the header tank for bubbles. When you get to the affected cylinder you will know it!