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!

__________________
Lance Allison
Lance@LanceAllison.com

Current:
11 MB S550 4Matic, 55k miles, Designo Black/Black
14 Ford F150 XLT Lariat Crew, 73k miles, 5.0
Coyote V8 4x4. Black/tan.
09 GMC Envoy Denali, 5.3 V8, 4x4 SWB. 38k miles,
Jewel Red/Med Gray.

Gone:
87 MB 300SDL, 320k miles, Astral Silver/Blue.
98 VW Jetta TDI, 488k miles, Classic Green/Gray.
85 Olds 98 Brougham FWD, 4.3 DIESEL V6, 80k
miles, 3x Gray.

MBCA Member, Chicago Region