Stumbled on this site and some threads about the S550 PP master, searching for spongy pedal issues. Read all the threads about possibly using S550 master, but didn't see anyone who had done it. So here goes. S550 has fittings on opposite side of, and one fitting larger than S197 (both S550 PP fittings, same as larger fitting on S197). S550 bolts up, but O-ring surfaces are different diameters/styles. I first tried both O-rings on S550 master, since O-ring surface is largher OD on S197 booster....... But it leaked air, so I eventually used some hi-temp gasket material and made my own gasket for entire MC mating surface. As mentioned in other threads, booster rod depth is drastically different. S197 does not use an adjustable booster rod, so I made my own out of stock S197 rod. Have to be careful of small steel rod sitting inside booster when removing booster rod. Need one new brake line between master and ABS block. I used NiCopp line as it would be a nightmare to bend that in steel. Was able to bend other line to opposite side and it has SS flex section, which helped some. Had to drill out smaller fitting for larger solid brake line, to use it with S197 ABS block. And thats about it, fits good, even if running a TVS (which I assume few are here) Fluid level warning is used for ABS/Advancetrac, so for now, just removed sensor from old master and plugged into harness, all happy....... Would love to tell you how great it is, but I've had a nighmare time trying to get air out. I assume it is air in ABS at this point, but have not gotten car to shop to have them do the ABS bleed.