2007 gmc sierra classic new brake booster and master cylinder

[Gabe Burch]

So I recently bought a new brake booster and master cylinder cause my old master leaked into my booster and so both had to go. I bought replacement parts, the exact same ones that were in my truck. Pn match and everything. Everything bolts right up. The problem is the master cylinder wont sit flush with the booster. There is a gap and the O ring on the master cylinder isnt even touching the booster. It's a vacuum booster so I'm pretty sure it needs to be sealed. Anyone every have this problem? It's a 2007 gmc sierra classic, vacuum assist booster, drums in the rear, disc in the front, etc....

[Mikebtte]

You should have an adjustable tip on the needle inside the booster. If it has a cap on it or it's adjusted out all the way it might make it difficult to install. Also, some Master cylinders come with a pin inside of them and if the old one had no pin them you need to remove it from the new master cylinder. 

The pin might just be a cylinder shaped chunk of plastic or metal or it may not be an issue at all. Some manufacturer don't use or offer it and some just put something in the end for shipping.