Educate me - Vacuum Leaks

Good morning, all.

I am working on my big dumb van this weekend, switching the vacuum brakes over to hydroboost.  Pretty simple swap, just get all the hydroboost items from an E-450 and bolt them in.

Where the challenge comes in is retaining the vacuum actuated HVAC.  The vehicle is a diesel, so it runs an engine driven vacuum pump to run the brake booster and hvac controls currently.

I bought the Dorman version of the F250 Diesel vacuum pump (the trucks use an electronic vacuum pump for HVAC and vacuum hubs), wire it in, life is good.

But the damn thing wont shut off.  It shuts off if you put your finger on the outlet, so thats working.  I plumbed in a reservoir and check valve with no change since I didn't have one originally and the F250 vacuum system has both a reservoir and a check valve.

So there is a leak somewhere.  I can follow the vaccum lines, stopping them off and getting the pump to kick off, right to the firewall, and then the line that goes from the firewall to the HVAC controls seems to be my source of trouble.  Stopping the line off all the way at the HVAC controls does not shut the pump down.

I am thinking I probably need a smoke machine to diagnose the leak, at this point I am running out of daylight so I am just going to put the functioning vacuum pump back in to run the HVAC, but really want this fixed at some point.

One other thing to note - the vacuum system actually works fine, the dash vents open and close happy as can be even with the small Dorman pump, but there must be a small leak somewhere.  

Any help?  Suggestions?

Vacuum leaks suck.

I'm here all week.  Tip your wait staff!

It sounds like you know your issue, it being the line from the firewall to the heater control.  Trace as well as you can, and then listen carefully.  If you can get everything else quiet enough...  Smoke machine would sure help.

Either replace all the hose, or use something like needle nose pliers to reach in and pinch off a suspect line, moving along its length until the problem doesn't go away. "Most likely" the leak will be at a connection point, but if you already found that it's upstream of that/those, try the pinch-off method.

Some systems use an interior temp sensor that pull in cabin air using engine vacuum instead of an electric fan.  Not super common but they do exist.  Could be a source of vacuum leak that does not effect operation. Do you have a hand pump that you can use to check how well it holds vacuum?  There is a good write up on instructables about building a smoke tester

In reply to Caperix :

The only problem with smoke testing a vacuum system is if there is a check valve, which most HVAC systems do.

The vacuum line likes to break.  Or if this has the ball shaped reservoir under the van, the hose breaks there or pops off entirely.

I've given up on finding the leak and just ran a new plastic line.  Oil pressure gauge plastic line works well for this.

Buttoned it back up with the high capacity belt driven vacuum pump and runs tip top.  

Will get into this later, need some more time for troubleshooting and this weekend was not that time.  

Thanks for the replies.  Good point on the check valve.  I do have good access to both ends of the vacuum system, so I could always introduce vacuum at the dash end too.  

What does the hydro boost work off of?  Do you add a new hydraulic pump somewhere or does it branch off of a power steering pump or something?

Hydroboost works off the power steering pump, E-450 had it from the factory, E-150 to E-350 had vacuum brakes.  

The swap is mainly a matter of just getting the hydroboost components from an same year E-450 and installing them.