I had a similar problem with my 603 motor. The system held a LOT of pressure, sometimes for days. Turns out I had a cracked head. By the time the new head, gasket, valve job, etc were installed, I was looking at a $5k repair bill.
I hope you are not in the same boat, but if you are, just remember once it's fixed, it should stay fixed for a LONG time.
Best of luck!
Sold: 1987 MB 300SDL, 320k miles, Astral Silver, Blue Leather, all books and records from day one.
Totalled: 1998 VW Jetta TDI, 488k miles, Classic Green/Gray Leather, loaded, E-Codes, "Slightly" modded, "Slightly" faster than stock, arrived 11/07/1997.
MBCA Member, Chicago Region