I went through a similar situation recently with a 103 cooling system.
It turned out to be a leaking overflow tank. It's pressurized and because it was not holding pressure, it was leaking as it would if the cap were weak. It's also possible that your cap is bad in spite of it's recent replacement, or you "might" have air in the system. These are but a few of the possibilities.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.