I know that for the 123. The power steering pump has a pressure control valve in the assembly. Not very difficult to remove and replace but I'm not sure on the 300E. Mercedes tends to use the same pumps on all the cars so if it is the same. The pressure regulating valve is in the pump.
The pump does provide a bit of pressure, I don't remember the PSI for it, check the manual, so it would cause a loss of fluid quite rapidly, but it doesa sound like you pressure regulator has gone bad for it to blow a seal in that fashion.
You will have to attach a pressure guage on the pump and measure it. Or just replace it.


Again my knowledge is for the 123.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car