There's no leak-up or leak-down, except for the leakage out of the coupler(s) directly. Not all couplers leak as badly on either machine.

I've got a 4640, with three ISO (pioneer 8010 tip-type) couplers, and a 4440 with two JD-style couplers.

The JD-style couplers leak a LOT less than the ISO style on the 4640. 

Oh, and there is this: the most commonly used couplers have issues with retention of the hose tip in the coupler. On the JD-style coupler on the 4440, we use a bungee cord to hold the little lock levers in position, and on the 4640's ISO's -- feh. The first SCV really, really needs to be rebuilt. It won't hold a tip well and it leaks pretty steady.

Are there instructions in the shop manual on how to rebuild these? I've been considering getting the shop manuals for these machines and just this weekend, I notice that the CD-ROM version is $100 and the printed manual is $200. Woof. That's a pretty expensive wad of paper.... any thoughts on how well the CD-ROM version works if you have to print out a few pages to take under the machine on a messy job?

thanks guys...