'88 GTA
From searching, I've determined that an LS1 master cylinder should be pretty much a direct bolt-in for my car. However, I can't find anything on the booster.
Will the LS1 master cylinder fit a third gen brake booster? From what I can tell, the bolt centers don't look any different.

 

Bolts right up. One of the outputs (front I think) is a slightly different pitch, but mine threaded in fine and doesn't leak.

Kevin D.

 

Is there some benefit to doing this ?

 

I don't know about any benefit. I just want it for the screw on reservoir cap. With the stock flat capped one that came on third gens, I can't get a good enough seal to pressure bleed my system.
I can only get 10psi out of it before the cap itself leaks or the grommets between the cap and cylinder start leaking. The service manual says to bleed it at about 20psi.