Best deal on servos is the rebuilt OEM unit for $295 from George Murphy (Performance Analysis Co.) at 865.482.9175. You should also pop the $50 for a rebuilt amplifier. I put these in my 380SL this week and they brought the system back to life.
You can bypass the servo with a 5/8" hose coupling from Home Despot, connecting the coolant feed and return lines.
George will also send you an excellent guide to troubleshooting the system with the rebuilt servo, or for $5 by itself.
Trivia - this system was developed by Chrysler and licensed by Mrecedes. Chrysler bailed in 1978, but MB kept using them until 1981. You read the description and marvel at how it ever worked.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe