I am installing a new master cylinder , but I thought there should be a seal on the back of the master to maintain the vacuum within the chamber , I know other cars have "O" rings and gaskets I was surprised when I did not find a seal on the 1800s brake booster so I thought I would ask I might just make my own thanks again 142 GuyOn my 142, there is no gasket between the base of the MC and the booster housing. Could be different on an 1800; but, I doubt it. There is nothing to seal back there.
If you ask because you are leaking brake fluid out around the mating surface, its probably time for a new master cylinder. Don't attempt a rebuild. New ones are available fairly cheap from the various Volvo specialists (Iroll motors, VP auto, High performance auto, maybe IPD, scandcar, CVI) and sometimes if you luck out, Rockauto.