Thats kind of a tough question to answer because there are a lot of master cylinders that can be used. The only way GM sells the grommets is to buy the reservior. Part number 18013404 was used on 82-87 G-Bodies so you might want to start from there. Wholesale on this part is about $25.00 so it might be better to buy a rebuilt M/C which comes with a reservior. The last time I bought a rebuilt M/C from AutoZone it was about $20.
Eric Fisher