Tracing vacuum leak in heating circuit
 

My '63 Bonneville (non-AC) has a vacuum leak which appears to be somewhere behind the tee coming off of the carb. I took the easy way out and initially swapped the vacuum diaphragm outside, but that wasn't it.

Next, I put my MightyVac on the line coming from the tee, and pumped up to 5lbs, which went right out. My next test was to disconnect the line from the inside diaphragm, plug it, and try again. Still, I couldn't hold pressure.

What role does the temp control knob play in this? My concern is that the leak is in the control in the dash, but I remain hopeful that it's just another 60-year-old hose.

I also haven't taken a deep dive into the shop manual, yet, but that's on the list.

Some guidance or insight into this type of issue would be greatly appreciated.

TIA

You can use a smoke machine to find vacuum leaks. It puts low pressure smoke in the system and you just look to see where it comes out. Maybe you could rent or borrow one.

There is a vacuum "switch" (well not really a switch but something) as part of the heater controls which determines, based on switch position, where vacuum is directed. It can over time leak and not hold vacuum and is nearly impossible to fix or find a replacement for. For a 63 vehicle I would start by taking the line from the carb to that switch off and check to see if it holds vacuum. If it does, then replace it on the switch and remove the air door and defroster door vacuum lines from their respective diaphragms and plug them. Then check and see if the system holds vacuum. If it does not hold vacuum, then check each vacuum hose for leaks. There may be some as it is an old car. If they are bad replace all vacuum hoses. If they are good, then likely the problem is the vacuum switch. If that switch is bad call Vintage Air or one of the others and replace the whole system as you will not likely find a replacement for the switch.

Great advice (both of you). Thanks so much.

I was concerned that the control would be the problem and how to fix it if it were. I'm working with D&M Restoration in Greenville, SC for the gauge restoration for my '56 Safari. If the control does turn out to be bad, I'll check with them to see if they have the ability to rebuild it.

Still holding out for a 60-year-old, dry, cracked hose (the one under the hood at the diaphragm there slid right off - too easily; I snipped the last 1/2" before putting it on the new diaphragm).

Smoke machine, eh? I'd never even heard of that before. I'll check and see what I can find. It might help me cut to the chase.

Cheers

First, the smoke machine is generally used to find leaks in the emission systems on newer cars but works great for any leak. Second if you are talking about the water control valve they have been rebuildable in the past. I found the company I used in Hemmings.

I went through having to replace the entire heating vacuum system on my 63 last year. I got lucky and found a NOS vacuum valve that goes on the control unit on ebay. I also bought 2 used heater controls to rob other parts from. Between that and all new lines and replacing both vac actuators and a new heater core i am all buttoned up.

Excellent news. Thanks for sharing.