First of all, thanks for asking this question to Hope4U. Okay. I went over to ABS Brake today at lunch and talked to Doug. He said to just use mineral oil. (I won't tell you what I put in mine because I was so far off, it's embarrassing.) So, all you guys who made that suggestion were right. He said they use an assemble oil but that it's a non petroleum mineral oil and to just use that. To his credit, he could have sold me an expensive bottle of assembly oil. BTW, there was another Hydrovac unit like mine sitting on the cart of rebuilt brake masters so I can assure you, they know that unit.
