A search "SLS valve" should give you all the information you need,if these links do not help.

SLS Valve in 190E 2.3-16v defective ???
http://www.mercedesshop.com/shopforum/showthread.php?p=731971#post731971

2.3 16V self leveling suspension troubleshoot
http://www.mercedesshop.com/shopforum/showthread.php?p=452801#post452801

Ups & Downs Of Self Leveling Suspension?
http://www.mercedesshop.com/shopforum/showthread.php?p=845940#post845940

Self level suspension hose warning
http://www.mercedesshop.com/shopforum/showthread.php?p=795040#post795040

Rear leveling suspension
http://www.mercedesshop.com/shopforum/showthread.php?p=162733#post162733

Self Leveling Rear Suspension Fluid Replacement
http://www.mercedesshop.com/shopforum/showthread.php?p=482947#post482947

Leveling sustem question
http://www.mercedesshop.com/shopforum/showthread.php?p=579948#post579948

SLS Spring Conversion Kit Interest?
http://www.mercedesshop.com/shopforum/showthread.php?p=1008542#post1008542

self-levelling height adjustment for w123
http://www.mercedesshop.com/shopforum/showthread.php?p=551723#post551723

re: rear hydraulic on 300td
http://www.mercedesshop.com/shopforum/showthread.php?p=856343#post856343

300TD SLS pump removal and suspension conversion
http://www.mercedesshop.com/shopforum/showthread.php?p=875528#post875528

w124 SLS valve on w123 wagon?
http://www.mercedesshop.com/shopforum/showthread.php?p=1109181#post1109181

Help understanding SLS/hydropneumatic
http://www.mercedesshop.com/shopforum/showthread.php?p=718381#post718381