I have been trying to trace down vacuum leaks in my 79. It has a 270 duration cam in it and pulls about 12hg at idle. My biggest problem is that the AC vent doesn't switch from heat vents to dash vents. My headlights come up fine (a little sluggish but not bad). The big bar vacuum storage tank in the front has a leak but I can't find it. I tried putting compressed air into it and it slowly bleeds down I have the out line to the headlights block so it definitively the tank bleeding down. I found a big source of vacuum leak in the cruise control unit (which btw is not hooked up) so I plugged the vacuum line that went to it.

So for the question(s). I used some hemostats to block the little black vacuum line that goes through the firewall and attached one of those vacuum brake bleeder pumps. It held vacuum with the hemostats attached right at the firewall so the line on the outside is fine Next I attached the hemostats to the black line just before the vacuum control valve (which I had just replaced) and it won't hold vacuum. I have the gauge bezel out and would like to check the black line back to the firewall to find the leak. Everything is behind the plastic vent that goes up to center dash outlet vent. How do I get that out? Do I have to remove the whole dash to get it out or is there some trick?

 

Check the control valve in the console. The leak is usually there. Edit:

Is that the valve you replaced? Check it again.

 

Yes that is the valve I replaced and I have vacuum loss before the black line goes into it. So my leak is occurring somewhere between the firewall and the control valve. Last week I did the Smoke test and while some smoke came out of the valve (the reason I replaced it) more came out from behind the vent before the valve.

 

Well it is kinda working. I finally got it to work when I reach a stable RPM. I figure I am creating enough vacuum to operate the switch. Through the process of elimination I found and somewhat fixed the bigger vacuum leaks. I am thinking about putting a vacuum tank dedicated to the AC. I was looking at the Dorman little round ball looking thing. Has anyone ever used one of these? How big are they? Do they come with a check valve? If anyone has any suggestions on a different tank I am all ears. I did manage to get the vacuum up to close to 14hg by resetting the timing and readjusting the idle screws.

 

I found that by replacing ALL of the plastic vacuum connectors in the four vacuum systems along with the hoses on the engine side of the firewall, I resolved the majority of my vacuum problems in my '80 L48. Also, inside the cabin, I replaced the switch that switches vacuum on the back of my heat/ac control as it was loose and 'wobbly' and I assumed I was losing vacuum signal there. Also, I replaced the vacuum-controlled valve in the heater hose as it wasn't cutting the hot water off to the heater core and I kept getting 'heat' when I selected 'cold' on the vent. Oh, I also replaced the front vacuum tank with one from Dr Rebuild along with the inner vacuum seals on my headlight actuators (look like little witches hats with holes in the tops). I removed the accordion seals on the headlight actuators altogether as they didn't seem to do much more than block dust from getting on the inner seals. All vacuum system hose sets I bought were custom sets from Dr Rebuild. No vacuum loss and steady carb settings since I resolved the systems over 10 years ago. Steady lope at idle with a Crane Energizer cam (272 deg Adv duration). Lars-rebuilt q-jet and professional re-curve of the HEI mean I get plenty of timing when I need it and plenty of hg of vacuum on/off idle. I haven't had to touch the vacuum systems since. Oh, I also removed/blocked ALL of the thermal actuators on the air cleaner and intake as I no longer had to pass emissions. I keep them in my tool chest for the day when/if I have to return it to smog compliance. No cat converter, true duals with headers, H-pipe and super turbo mufflers. I replaced the plastic vacuum fitting on the power brake booster as it had cracked and was losing vacuum. Essentially, any plastic or rubber in my engine compartment vacuum systems was replaced with new. short of removing my dashboard to get at all of the small lines/hoses, I limited my interior/cabin vacuum work to the immediate area of the heat/ac controls in the console. Pleased with the results. Oh, I also replaced the vacuum canister behind the left front wheel assembly. With it replaced, it properly manages gasoline vapor recovery once the car is shut off.

 

If you want to find where the leaks are try using a smoke machine instead of just air. If you do not have a smoke machine the next best thing I have found is a cigar and a friend. Take a big puff and blow the smoke through the system and look for where it comes out.

 

I bought a vac pump from Harbor Freight to help troubleshoot my headlight issues. I also have one of the small brake bleeder hand pumps. I could then connect the hand pump and the vac pump and see what my system vacuum reading was. I could also test certain parts of the system to see if they held vacuum.

The AC control circuit was good. Everything worked there. I replaced the actuator seals, rebuilt the relays and replaced the filter and check valve. The hoses were fine, reservoir was fine. Now the headlights open quickly.

 

The system will only work properly when vacuum is applied at the correct location. Many otherwise healthy components will leak when pressure is applied so this is not really a valid test.

 

I have to dispute that. I do not feel one is applying enough pressure to force something to leak. Having used this method I have found it works very well especially troubleshooting the headlight and wiper vacuum systems.

 

I agree while there may be some forced leaks it is a great indicator. It helped be zero in on areas of potential leaks.

 

Whether your using pressure or vacuum be careful not to over do it,hd vacuum pumps can go to 29.9999 which might rupture seals. I limit mine to around 15".

 

That's fine, I was just speaking from personal experience. The check valve and headlight override switch are just two examples of components that will always show a leak when pressure is applied.