Have been perplexed by my 94 Z28 with 64k easy miles that was always showing couple of pints short of coolant every time I opened the radiator cap.Put a 20 psi test on it and fixed all the newly replaced hose clamps.The reservoir never moved up or down.Found that at operating temps some coolant was leaking around my new cap.So I replaced it.3 days later same problem.Had a rainy day today so did some testing.Through suction and compressed air no movement in reservoir.Pulled the battery and removed the reservoir and battery tray.Found blockage where rubber overflow hose meets the reservoir aluminum tubing.Took some heavy gauge wire and snaked it out.That line was blocked solid and took several tries to clear the line.I assume the blockage created such high pressure the shortest route was the cap.

 

Congrats!

It's always nice to find the problem and fix it. Thanks for sharing, maybe it will help someone else in the same situation.