My power brakes on my 86 rx7 n/a work only after i've revved the engine up, and dont brake for a while, then they work for about 4 pumps of the pedal, but still they are weak. After that, its driving without power brakes untill you rev it up. This seems like a vaccum leak to me, could it be a faulty booster? This problem only happened after i put another engine in it. Any ideas would be great.

 

oh yea, i checked the vacuum line goin the the lower manifold from the brake booster and that line is all hooked up. Anywhere else i can check?

 

The brake booster runs off vac only. Not much to it. I think there is some type off check valve in line on the on the line. Check it to see if it's suck. and free flowing.
If the line is on good (brake booster & mani) The valve is ok and the mani. is makeing good vac..........then that just leaves the booster it's self.

I may be off.....check to see.