When I changed my ATF, I also used my mityvac and pumped out the PS tank. Refilled with Pentosin fluid, ran engine a bit, re-vacuumed and refilled. Easy job! Wouldn't think it needs to be done more frequently then ATF change schedule every 40-50k.

__________________
-- Chris

'95 E300, 216k miles, Silver Surfer
'05 E320 CDI, 138k miles
'07 S550 4matic, 69k miles

Gone but not forgotten:

'76 300D, 350k miles?, SOLD in 1995
'75 240D, 300k miles, SOLD in 1991