I had the same symptoms with or 116 model diesel. The first place to check is the vacuum pump. No matter where other leaks are, if the pump is leaking fixing all the other leaks won't solve the problem.

Buy a MityVac vacuum tester; should be about $25. Disconnect the main line from the pump to the brake booster at the brake booster. Plug off any lines teeing off this line (golf tees work), connect the MityVac at the brake booster fitting and pump vaccum. The vacuum should both pump to over 15 psi and hold there for quite some time. If it doesn't pump up and stay, you need to rebuild the pump, which is not all that daunting. After rebuilding, then you can either have a working vacuum system or be able to troubleshoot the rest of the system.

Jim W.