Since the hose stays collapsed with the cap loosened (relieving vacuum on the system), the hose is softened to the point it's time for replacement, or there's a clog somewhere in the system that's preventing the vacuum from breaking in the main coolant circuit when you open the cap. Radiator hoses are cheap compared to the cost of engine R&R...

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)