'74 headlight vacuum question

[Tom Sarno]

I am redoing the vacuum system on my newly acquired '74 that has had all the hoses removed. I know the routing and understand how they work, but I was wondering if anyone knows how many inches of vacuum the storage tank should have when the car is running. If the storage tank is leaking, would a vacuum tank similar to the ones used for power brakes have enough volume to run the lights.

Thanks,
Tom

[Michael W.]

Re: '74 headlight vacuum question

In a system that is working 100% as per factory original, the vacuum in the tank would be equal to that measured at the intake manifold port. Anything less indicates that there is a leak somewhere.

As a leak grows larger, the vacuum level in the system decreases to the point that the headlights move very slowly or not at all. Changing the size of the vacuum reservoir in a leaky system will have no effect.

The C3s had a reservoir primarily to allow the headlights to raise/lower when there is no vacuum available (engine shut off) or low vacuum situations like WOT.

I doubt very much that the reservoir on your '74 is leaking, it's a very substantial piece and 'doubles' almost as a front bumper.

[Jim T.]

Re: '74 headlight vacuum question

When my 70 was new I could pop up the headlights the next day without starting the car. Sure can't do that today, all the vacuum components are still original except check valve and filter.