the "cap" is what is from the factory it is part of what people remove to do the actual diff breather mod, those caps will work untill you go in about 18 inches of water or so.
when i did my diff breather i actually did it a little diffrant than most people do. if anyone is interested i can get some pictures.
i first removed the top cap of the stock diff breather (don't have to pay for extra fittings that most people buy) and found tube that fit over i think it was about 3/8" hose irrigation hose i got it from home depot i attached that to the stock breather(minus the cap) with a hose clamp
i ran the hose from the rear along the frame rail/fuel lines/what ever i could afix it to and conected the 2 around the firewall with a t-fitting (muti size from autozone) and ran a hose off the 3rd part to the top of my fire wall and put a small "ricer" valve cover breather filter on top.
a point that some one may bring up is the hose. it is a plastic like hose that is used for irrigation i was worried about petrolum gear oli eating through it but after a year it is still 100% fine plus the oil rarely actually is in the hose maybe a splash now and then or vapors but it is not always carried in there
if anyone see's a problem with the way i did this let me know as i can not see anything wrong besides it is some what over kill on the size of tube and the way i ran but i also see it as 1 less thing that i have to worry about.