Does anyone know how to test a Door Vaccum system for 84 300D. My locks loose vac...I guess. After the car off for 20 min or so, my locks do not work. After I start the car and vacuums builds up after a minute or so door locks pop open (or close). And it's all the doors, including trunk, fuel door. Do I have a major vac problem?

It could be the check valve from the engine to the resovoir, it could be a leaking hose. There is a $50 handheld tool called a "mighty vac" it allows you to draw a vacuum, and a guage to see if it holds it. The system as I recall from my old 85, was a valve in the drivers door, operated by a rod from the key lock, which routed vacuum from the vacuum tank to the diaphrams to lock or unlock the doors, trunk, and gas door. The vacuum is derived from the engines vacuum pump thru a check valve.

Thanks Peter.
I do have a "mighty vac". Where do I test the sytem to find out where the leak might be? Is there a particular spot I should check first. Could you give some detailed directions, as I'm new at it.
Thanks, Alex

Start at the vacuum lines found at the firewall. This is assuming that the other vacuum accessories such as power brakes and shut off work okay.

Apply vacuum to the lines that go through the firewall and see if they will hold vacuum. If I remember correctly, the vac door lock feed line is yellow with a green or purple stripe. If it holds vacuum, then leave it applied and lock the door with the drivers door lock. Roll down all the windows before you start for convenience.

If it does not hold vac when locking the front door, then start tracing the lines and applying vacuum until you find the diaphragm that does not hold vacuum, that is your leaker.

Get a performance parts catalog and they have a vac diagram in them. At least they used to.

Good luck,

Thanks Larry
Yes, the rest of teh vac system is working fine.... engine shut off, power brakes.
I will try to find it the way you told me. I will let you know what I came up with.

When I lock the drivers door I can hear an air leak.... I opened the door panel and the master Vac Valve makes the sound of an air leak when I turn the key in the door. It sounds like there is presure being relased from the valve. Is it supposed to do it, or there is my problem with the door locks?

I thought the doors trunk and fuel door were all dependent on the ele. vac. pump (126 at least) in the spare tire well I'll bet if you remove the fuse for the trunk pump that the locks won't work by engine vacuum or stored engine vacuum that is stored for the brakes????

William Rogers..........

123 (diesel) used vacuum from the engine driven pump and did not have a electric pump.

Peter is right. On 123, the entire vacuum system feeds from engine driven pump.
The question still stands, could the master vac valave in the driver's door create the problem for all door locks, or should I look somewhere else?
Thanks

Yes you should not hear any hissing in the doorAs I recall that valve had two positions, in one it would route vacuum to pull in the locking diaphram, the other would reverse it. If it leaks it will deplete the stored vacuum and not be able to send any vacuum to the consumers (door, trunk, gas door)