Brakes hard to use, can hear hissing noise inside when applying
Is there a vacuum line inside?
Haven't checked check valve yet.

 

There is a seal where the pushrod goes into the vacuum can. It's bad. I've got the same thing in my 2000 WJ but only on heavy braking. I've lived with it for 5 years without changing it and it works fine on everything except panic stops. The braking action is okay, it just makes a whistling sound. Yours must be worse if the brakes are hard to use.

 

Happen to know part number for seal and directions to replace it?

 

Internal parts are unavailable. You're going to have to buy a reman. It's a safety issue.

rockauto.com has an ACDelco reman for $84.79 w/o master cylinder + $10 core + shipping just as an example.