Hi, sorry to bother you guys. Been looking at my sons 1999 Grand Cherokee. Under the master cylinder is a check valve of some kind?? One Pretty large (looks like vacuum) hose coming off of it. Ours is not connected? I've looked around but don't see where it connects??? Tried Google search for vacuum check valve / and many other searches before I asked here. The lose end is white plastic covered with oil only thing looked close was the transfer case control switch but couldn't find any info about this? I know you guys can answer this EZ question? Thanks Tony Looked again this valve is hanging on the top threads of the drivers side shock

 

It's the vent hose for the front diff.

 

Thank you, I will take a look

 

Crawl under it and on the rear upper driver's side corner of the diff, there should be a hole. The plastic piece press fits into that hole. When mine came out, luck would have it that it got squashed a little so it was tough going back in. I doubt it will EVER come out again. Check your front diff for water contamination and loss of fluid.