Burgie, looks like a nice truck! Garwood bed too, I have a 52 4400 with the same dump setup and two speed rear that is vacuum operated. Works well and has two hoses like yours. If the engine is idling and you shift from high to low and back to high again, you should note that the engine revs will go up a little due to leaning out of fuel mixture and you should be able to hear the vacuum "whoosh" sound for just a second and then it stops. (This of course assumes you got a good mufler and valves are in good adjustment so you can hear over the engine noise)
If you hear a constant sucking sound in high range, but not low, you have a leak on the high range side of the diaphram.
Good luck!
