We have these roughing "bypass" valves on all our roughing pumps:
http://smf.probesoftware.com/index.php?topic=120.msg487#msg487
Prevents thin window blowouts when pumping down from atmosphere.
My engineer reports lesker.com has the following:
KF25 size, pn 31128-KASO for $135 (only 1 left in stock)
KF40 size, pn 31132-KASO for $148 (4 in stock), and you'd likely need 2 KF40/25 adapters to fit these in your system
Here are some pictures of these slow opening rough values on our SX100 (and also the Sx50):
http://smf.probesoftware.com/index.php?topic=332.msg2077#msg2077