Ironically, this has happened to about 3 users in 2 weeks!
Bleed the pump and see if it stops. Your pump has air in it and the air gets compressed when the engine is running. Then when you turn it off, the air expands rapidly and wants to get out! So P/S fluid goes flying as well haha. Look around there are other threads on this. Are you leaking P/S fluid somewhere else too?
2008 Mercury Sable Premier
2014 Ford Focus Electric
1997 Mercury Grand Marquis GS
Moved On: 2000 Sable LS Premium Duratec