I'm afraid I don't know what you mean when you refer to a " directional valve".
The climate control system on a 1979 450SL is usually referred to as ACCII (Automatic Climate Control, version 2) system. In the engine compartment, up next to the firewall on the passenger side (US models) you'll find a black-capped tower with many electrical and vacuum connections. This is called the servo. Under the passenger side dash you'll find a rat's nest of more vacuum lines, vacuum switches, and screwed to the heater box, a small electrical board called an amplifier. The black grill on the top of the dashboard (if you don't see it, your car has a dash cap and someone has covered it up - you'll need to fix that) is the cabin temperature sensor. Under the hood, and under the blower cover - the large black plastic cover just below the windshield - is the ambient (atmosphere) temperature sensor.
When you press a button on your climate control panel - say Auto Lo - that is supposed to send a signal to the amplifier, which boosts it high enough that it can be read by the servo. The servo looks at the temperature you have set on the climate control panel, compares that with the actual temperature of the cabin, and then looks at the air temperature. If the temp wheel is set to 75, the cabin of the car is 65, and the ambient temperature is 45, it opens a valve, allowing coolant to flow through the heater core. It also sends vacuum to the pod that opens the footwell flaps, more vacuum to close the defrost and center vent flaps.
If the temp wheel is set to 75, the cabin temp is 85 and the ambient temp is 95, it closes the valve for the cooant, closes the footwell flaps, keeps the defroster flaps closed, and opens the flaps for the center dash vents. That's the way it's supposed to work.
Unfortunately, the servo/amplifer is an overly complex, badly designed piece of mechanical history. MB only used it for four years (78-81) - and since it takes about a year to decide it was a bad idea, and another year to redesign and roll out the replacement - well you can see it was a problem right from the beginning. And when the servo goes, it tends to take the amplifier with it.
When the servo fails (and it really is 'when', not 'if') it will get stuck in whatever mode it was in when it failed. If it was supplying heat, you get heat all the time. If it was supplying cool air, you get no heat, ever. A rebuilt servo, from a reputable company that offers a 1 year warranty, will set you back $400 to $500. A new amplifier will add another $100 to that.
There is a company called Unwired Tools (I am not affiliated with them in any way) makes a digital replacement for the servo/amplifier. It will set you back about $700, but it lasts the way the servo won't. I've had one in my 1978 450SL for two years, and it's been flawless. Before that I went through three servos in two years. If I had it to do all over again, I would have bought the digital replacement right at the beginning, and saved the $900 I spent on servos and amplifiers.