You can't go wrong with PRM skids. I have them on 3 ATV's. They are heavy duty.

The 85-86 250R is a dual piston caliper. Not sure if it is worth it to try to make it fit on a 350X though. Are you running large tires or something? I run 22" All Trax on mine, and the brakes seem to work fine.

 

I'm not familiar with the rear brakes on the 350X, but I would be that you need to rebuild your back brakin system if it doesn't work to your satisfaction. I don't recall Honda building any ATV's with inadequate brakes. There where a few years when the dirt bikes where a little behind the curve, but not too much.

I'd get a new set of seals and dust seals for the rear caliper and
a rear master cylinder rebuild kit and a braided brake line and install them and see how it does. My 250R with rebuild brakes (which is roughly the same performance level as the 350X) stops just fine. If you decide to do the rebuild route, just ask any questions that you happen to have, as I've done it to the R.

PS-Since you've got an 85 R, you should be able to see the differnece.

 

now that you said it i didnt even realise my R had thoes breaks.and i was just wonderin if a dual piston one would work better mine work really good now. it was just a thought. i get alot of thoughts you know how that goes some good some bad. lol