Shifter leaking from handle but only in high range

I have an old 1981 international with a 9 speed Eaton transmission with a crappy shifter. I got a replacement shifter from a friend’s 18 speed with a range selector and a gear split switch. I figured I could swap it out and only connect the range selector lines no problem but now I get air leaking out from the shifter knob but only in high range. The range piston moves just fine though. The old shifter didn’t leak at all, it just had an awkward selector switch and a trashed boot that let exhaust in the cab. Any idea why it would only leak in high range and only from the handle?
Thanks

 

I took apart the head and didn’t find any problems but when I put it back together it now leaks in both ranges. So I decided to combine the parts that work from both shifters and got something to work. It’s not great but it’ll drive. Thanks for the suggestions everyone.