Hydraulic clutch repair work on my 1966 GMC 5500 truck first led to replacing the master cylinder, and has now expanded to the vacuum system that ends up connected to the hydrovac. This inter-connected vacuum system includes the rear end over-under (high/low range) switch that is controlled by the shift lever button. In addition to connections on the switch that go to the hydrovac and the rear end, there is a line that ends up at the left side of the dashboard near an obviously replaced vacuum gauge (salvaged from dairy equipment) that is not working. This line is unconnected (open) at the dashboard end, where I am guessing that it is supposed to be hooked up to the vacuum gauge. I am hoping that someone can tell me if this guess is correct or not? It just seems odd to bring a line from under the truck to measure vacuum.

Right now, I am trying to track down other vacuum leaks, including those coming from the rats nest of old hoses around the hydrovac. And I am replacing the brake booster check valves after breaking an end off one of them. This gave me a sinking feeling because parts have been hard to come by, but it looks like Napa has what I need.

I also have a story about finding a new tandem (clutch and brake) master cylinder to fit this truck that I will share after I know that what I ended up with works.

JRMunn

JRMunn