2002 Sportsman 400 Bad Front Master Cyl.

[Dominant Predator]

Ok so I have an 02 Sportsman 400. I have been up and down the line looking for a leak and have come to the conclusion that the front master cyl is bad. I can't tell if its the cap is letting air in some how or the whole unit is broke and letting fluid pass the seals. Im not loosing any fluid but if I go to apply the front brakes they work and then like loose pressure. Like if I were going down a hill, I pull the lever and its got good resistance on the lever and the brakes work. BUT if I hold it for more then a few seconds it looses pressure on the lever, it pulls all the way in and I no longer have front brakes. If I let go of the lever and reapply the brakes come back and then it happens all over again. This does not only happen on a hill I can be flat ground or even not moving like when I put the parking brake on so I know its not the brakes heating up or anything like that. I can't seem to find a front master cyl for less then 120. Can I use one off another make ATV? I know it will have to have the banjo bolt style hook up for the line but other then that it should work fine right? Any help would be appreciated either how to fix the one I have or going with a new unit, most of the ones Ive seen are like 30 bucks why Polaris needs 120 for theirs is unknown to me. Im on a TIGHT budget and really would like to have brakes. Thanks in advance.