Heats at idle but not while driving

[bluebird]

I have a 1984 300SD, the car heats really well while idleing but when I drive I recieve very little heat either at defrost or max heat settings. Could this be the monoblock diaphram? If this is the case does much coolant leak out after pulling the 4 screws out of the monoblock and removing the diaphram? In other words should I clamp off the hoses going to monoblock to prevent the coolant loss? Thank you for all the excellent information. I don't want to tackle this whimsically as I just had a complete Knee replacement 1 month ago and it still hurts like he## when I strain it.