Hi to any who may know,  I have a new power steering slave cylinder installed. It has fluid in the system. Has never had pressure on it from pump. Car is in rebuild stage. It leaks just from setting still. When I get the engine ready to run, and pressure on the system from the pump,  will the seals in the rod seal up from pressure and stop leaking, or do I need to replace/rebuild the cylinder. Thanks. 

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I had a similar experience during my restore. I bought a reman slave unit that leaked from the get go. When I got tired of fluid on the garage floor and pulled it apart, I found that one of the seals was folded over (I.e., poor quality control). I returned it for another, which has been dry to this day. 

I doubt pressure will make your problem better. My guess is it will just make the puddle larger lol. You probably need to pull it out and see what’s going on.