1991 F150 5.8

I'm essentially wondering how it works and when. And just to be clear, I'm talking about the vacuum reservoir, not the charcoal canister.

Even after replacing all the rubber vac lines on my truck it's still got a vac leak somewhere. I replaced the lower intake gasket a few months ago and just to make sure it wasn't leaking, I've already checked the mating surfaces w/ carb cleaner- no leaks.

But I did finally get around to buying a vacuum gauge/pump. I got the truck up to temp and then killed the motor. Using my vac pump I began checking the different connections. When I got to the line that tees off to the servo/vac canister, I noticed something odd.

The line runs from the vac tree on the upper manifold, thru a check valve, then tees off to the servo and vac canister. When I tested the line behind the check valve [opposite the manifold], it held vacuum. When I tested the line that runs to the servo, it held vacuum. When I tested the line to the vac reservoir, it did not hold vacuum.

What's weird is, when I first pulled the line loose from the back of the check valve, there was a quick rush of air into the line, which tells me that the vac canister was holding vacuum. So why, when I tested the canister with my vac pump, didn't it hold?

And yes, I'm sure I had a good seal between my vac pump and the line.

As always, thanks in advance.

 

The purpose of the vac canister is to hold some sort of vacuum while you throttle the engine (where it produces little or no vac until RPM increases). This is used to hold vacuum operated accessories in place until vacuum returns (A/C vents, controls and such). You could have a tiny hole in the vac canister. If you have the soup can style, they are prone to rusting out. The reason that it might not hold a vac might depend on a couple of things. How did you plug off the other port on the the canister during the test? You might have a hole in it small enough for you to detect vacuum degredation with a gauge but when you pulled the line, there was still noticible vacuum, just not full vac.

 

I had the juice can on mine. It looked good but leaked. When I removed it from the inner fender; there was no bottom on the can. I replaced it with a round plastic reservoir from LMC truck.

 

Mine had a lot of pinholes rusted in it although it was still structurally sound. I wrapped it end to end in black electrical tape, overlapping each pass over half of the previous one. I meant to eventually replace it but never got around to it before I had to retire it for other causes. It worked for me but as always YVMV.

 

Yeah I've got the soup can style on there.

The other port is plugged at the rubber connector. There's just a plastic connector there with a cap on it. It's been that way since I've owned the truck so I just assumed it was factory. But maybe there's something missing I don't know about?

I suppose my next move is to pull the vac can and look it over. If I can find something to plug the hose with, maybe I'll try unhooking the can completely just to see how it runs. If it smoothes out then I'll have found my answer.

Thanks for the explanation/advice.

Edit- I just went out and plugged the line to the canister with a bolt that fit the hole. I'll see how it runs on my drive to work and go from there.