I have a 97 jeep cherokee sport. I swapped a ford 8.8 in the rear for strength and the disc brakes. I'm havin trouble getting my brake pedal to pump up. I've bled them out numerous times. I have a little bit of brakes but nothing even close to bein normal. Do I need to modify anything on the master cyclinder to accomadate for the disc brakes on the back?  

Yes, when you went from drums to discs in rear you changed the volume of fluid needed for rear brakes (you need more volume) Your current master cylinder is not up to task. You need a master cylinder designed for rear disc brakes.  

I had buddy tell me there's  a valve in the master cylinder that I could swap out with one from a grand cherokee that has disc brakes in the back. Is that a possibility?  

Never heard of that but is has to do with bore of cylinder for rear circuit. Discs need a bigger bore to displace more fluid to them. (this is why it feels the need to be pumped up) You need to change master cylinder.  

Do u have any idea what kind of master cylinder I would use for that?  

I would suggest that you try one for a Grand Cherokee that has rear disc brakes. It need to be compatible in mounting and fittings.  

I appreciate the help snoman