Hy hope4u, the old hydrovac has a felt backing up the seal proper, I was told that the lube served two purposes, one to keep the inner surface from rusting, the other to keep a line of lube at the edge of the seal. As you probably realize standard lubricants do not perform well in a vacuum, that is why the vacuum cylinder oil was developed. I would not use hydraulic oil myself, I know that others on this forum have tried to find the correct lube with little success, I would suggest using an oil recommended for a vacuum pump, the type of pump that diesel powered vehicles use to generate vacuum for their power brake boosters. As you known " brake fluid " is not an oil at all, and any " oils " that enter the brake system cause no end of problems, hope that helps
