I set out this afternoon to fix the loss of dash vents during acceleration.

I tested all three of the servo on the drivers side including the dual acting servo. All of them held vacuum with only minimal losses over the course of a minute. Just for sport I plugged and clamped the yellow line - no luck.

under a hard long acceleration I still lose the vents. talking like 15 seconds at 4,000 rpm, think climbing a hill, fully loaded with a cargo box on top.

I checked the check valve with the blow and suck method as well as with the might vac. Its solid. I checked the tubing system from the manifold to the firewall including all the rubber elbows and unions, although the have some surface cracks, the entire system held vacuum fine.

I understand how the system works: if the yellow line or its servo has a leak , the single acting servo that controls the dash vents will lose power and will default to the feet.

What should I be checking next?

Does anyone have a schematic of the vacuum system??

Could I run a line directly from the cabin side of the check valve to the dash vent servo control as a diagnostic?
Thanks,
matt


--
'90 245 285k, '93 945 296k

oh my this is frustrating, I pulled the vacuum reservoir and checks OK.

What are my tactics for diagnostics?? I'm thinking I'm going to have to T a gauge into the system in various locations and then go for a drive because you just cant simulate it at idle.

I tried to get the MCC unit out to check the connections (which I've done many years ago) but two little metal tabs on the bottom of the unit kept it in there. It looks like I need to remove the radio.

What if I added an additional check valve immediately upstream of the dash vent servo? If it works.. I'd be stuck with dash vents only, but at least they would work -- remove the check valve after summer.
--
'90 245 285k, '93 945 296k

I did some googling and came up empty on a 940 vacuum diagram.

Could it be a seal on either side of the firewall check valve or intake manifold? If the system is holding pressure fine, then the leak may be the lines themselves. A long uphill slog with heavy throttle might create enough intake manifold vacuum to allow air into the lines through a small crack.