i Just bought a 260z datsun with a new motor and tranny. Brakes are soft and it takes a long time to stop. every time i hit the brakes my rpm goes up so i am assuming some seals in my brake booster are leaking causing a vacuum leak. so i will need a new brake booster. i checked all break lines. no leaks and fluid is topped off.

Welcome! If that's a question to break booster gone bad I would say yes. 

Thank you that's what i thought.

Also worth looking at the vacuum hoses for it, a leaky one would also effect it.  I just installed a reman booster and new master cylinder not too long ago.  Might be worth the peace of mind if your MC is suspect.  Rear wheel cylinders are also notorious for leaking when old. FYI

I would have thought a vacuum leak would result in a drop of rpm (rough running) but I work on EFI more that carbs so maybe missing something. but yea, bad booster, you could always just put a vacuum on in and see if it holds. The booster can fail in odd ways, mine leaked around a seal so only when I touched the brakes, others may leak constantly.

I think it depends on how rich or lean the mixture is.  If it's rich the extra air raises idle speed.  If it's lean or close to "right", it would drop it.  Also depends on how much air leaks in.  But the change alone, with brake application, is the clue.

You may be right, @Dave WM , I just know that little vacuum leaks can wreak havoc for some components, which is more likely on old/brittle lines.  I would certainly replace the associated hoses while I was at it, if it is affordable.