You're thinking of the EGR. The PCV (breather hose) is the plastic pipe that runs from the valve cover next to the oil filler cap down to the rubber boot at the turbo inlet. It is to suck fumes from the crankcase into the intake air to burn instead of just venting.

Regarding the EGR: Leave it alone. On these cars I doubt you'll see any real difference in emissions with it connected vs with it disconnected. At the age and mileage of these cars now, there are other factors dictating how "dirty" their exhaust is. What will happen if you reconnect the EGR is a gradual sooting up of the intake manifold. Over time, the engine will lose power and burn MUCH dirtier, producing more soot, and rapidly clogging the intake. My SDL was so clogged with soot when I got it that the intake runners were down to 1/2" holes bored through a solid mass of soot and goop. Talk about a mess to clean up!

Before I'm taken down in flames regarding the EGR as an emissions control device, I believe in emissions controls that work. EGR is a half-assed attempt at controlling emissions on the cheap. It may work fine when the vehicle is new, but over time it causes more problems than it solves, even on gas engines. Put some miles on the engine and you wind up with soot and buildup on the valves and intake manifold. If you don't think that increases emissions, think again.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]