Much good information here as I have recently began experiencing mysterious loss of coolant too.
I have only had chance to take my car out 3 times in last 4 months as I have been slowly replacing clutch master, slave and assorted odds and ends. First drive 4 weeks ago was beautiful, 4 trips @ 1/2 hour per, car never sat long without running, ran cool and smooth all afternoon. On start up I did hear a minor squeal/grind that I felt was beginning of water pump failure, (oh well) but it went away quickly, outside temp 75F. Two weeks later open garage and notice considerable amount of coolant saturated into cardboard under car. Checked the usual suspect areas at front passenger side of car which cardboard seemed to indicate leak from and found a slightly loose lower rad hose clamp, tightened, filled rad, started and observed nothing for 30 min. Problem solved? Nope. Yesterday was 55F and I took it on a short hop of 2 miles and noticed a wiff of coolant on start up. Other than a bit of sluggishness with cold lubricant at first, no problems warmed up. Today I wanted to button her up after a short drive @ 57F. Started ok but noticed a distinct, intermittent smell of coolant as I managed to barely make it around 4 blocks without stalling. Upon return, I had difficulty with clutch sticking, some white smoke (very little) and what I think was a combo of Lockheed fluid and coolant (definitively coolant). Also seemed engine not developing full power, like a brake was dragging. Did not notice fluid leaking from system or on y pipe but I have had to top off again once. I did observe a tiny bit of coolant at top of temp sending gauge.
Engine block was boiled, fluxed, bored .40 over. Excellent cylinder head core (thanks Agm) was rebuilt by the best shop in town summer of 2009. I did check torque of head bolts at 500 mi. I feel pretty certain that the integrity of the block and head is sound. The car does live in an unheated, 100 year old garage but it is a very sturdy structure and coolant/antifreeze protection exceeded temp. And I put about 500 miles on it this summer with no issues.
I know from reading these posts that the big job is finding source of leak. But I have one more wrinkle to add. I turned the heater on (a first) during trip one to see how it worked. That is the only apparent variable. I did not notice any coolant in cabin and did not observe typical core set up when I removed engine. I have put over 1,400 miles on the rebuild with the typical component resto issues but nothing major.
Sounds like I have at least 3 or more distinct (Sunbeam) issues, but was hoping impartial cooler heads may provide some additional insight. I have learned so much from you all and the good old WSM.
Best Regards
PS: This car (B94102157) has a very old SAOCA decal in window and is listed in registry with it's original color. It now appears to have a 30+ year 2 stage enamel job over original Med Blue. I have been able to trace to California in 2004, (see photo) I picked it up with a blown piston/head in Baldwin, KS in 2008. Anyone recognize it? Is there any way to trace back further through club records?