Vacuum lines and exhaust leaks are a pain to diagnose on automobiles. Companies have come up with a kit to test these systems with smoke, but they can cost upwards of $750.
By using the cheapest smoke machine I could find on Amazon ( https://amzn.to/2HCGdjV ) which was about $26 plus $10 for fluid ( https://amzn.to/2sSLONV ), and a few cheap parts I had laying around, I have made a machine that does everything their machine does for a fraction of the price.
There are two parts:
First is the main adapter. It is designed to not have to modify the machine in any way. A simple automotive O ring goes on the back side (I sized it to one I had handy) and the end fits a 1/4 in ID x 1/4 in MIP Hose Barb Adapter. You will probably also need some screws that are about twice as long as stock. A pneumatic hose can be clamped onto the barb, and a female quick disconnect can be put on the other end of the hose. I recommend Harbor Freight for those parts.
The second part is a cone that has an opening to put in a male quick disconnect fitting, and the other end can be put into any hose up to 1" in diameter. Additional adapters are easy to make. Just make them bigger, such as for exhaust.
Use is to disconnect a line at the end of the vacuum on your vehicle (such as the break booster line), insert the cone, turn on the machine and look for leaks. Any leaks that are visible will need to be dealt with depending on the parts in question. This prevents the need to systematically replace parts until the problem is fixed.
It is a fun project.
I use a New Matter Mod-T with Cura. The parts were modeled in Fusion 360. I have created the parts so no fill material is necessary, and I did not need a raft for proper adhesion on any parts either.
***A couple of notes: The screw holes were too small on purpose. It is much easier to drill them bigger to fit, than to have them too big.
The hole for the threaded barb adapter is also VERY tight. I beveled it to help get the adapter in, but it was right at the point of breaking the part when I screwed it in.