1972 AC Vacuum Line

[KellerMAC]

My heater control valve will not close when I turn on the AC and I am in the middle of trying to determine why I don't have any vacuum coming out of my A/C slide switch under the dash. It appears that I have some going in but nothing coming out. The vacuum going is very week. So, I am going to put a vacuum gauge on the various points throughout the system and was wondering if anyone could tell me what readings I should expect on the vacuum gauge? Please see my picture below and if you can lend any insight, that would be greatly appreciated.

BTW, I am running a two port off the manifold - one is going to the brake booster, the other has a T that is feeding the transmission and the HVAC unit.

Thanks in Advance,

KellerMac

[SS Tim]

Nice to see someone trying with a diagram and all.

A hand vacuum pump with a gauge is your friend if you don't have one, buy one.

First I really think you have leaks from your description. The easiest place to start is at the valve itself, have you tried cycling the heater valve with a hand vacuum pump? Does it hold vacuum? After that disconnect (and plug) at point H and pull a vacuum at F. It should hold a vacuum. If it does not then you will need to further break things down by section and see where it is leaking. Remember it can be a hose, an actuator or a tee. Golf tees work great for temp. plugs and you might need a section of hose to block something off or jump for testing. The control switch may be the root cause but I would eliminate the system by leak testing first.

Second you think the source vacuum is low. Disconnect the line at the manifold and put a vacuum gauge on the fitting (vehicle running). Reconnect and check at the switch C. If it is not the same, there is a line or reservoir leak. Again plug and test with a vacuum pump between A and C. Then break it down from there. Like C-B and B-A.

[KellerMAC]

Thanks Tim,

Great stuff to try - Should I get the same reading coming out of the Vacuum can as I have going into it or does the can regulate the outward vacuum?

[SS Tim]

Not sure and I don't have one to check here. IIRC there is only a check valve in the vacuum source side (engine) to prevent bleed down.

One other thing is your tee for the vacuum modulator. Is it big enough for both the A/C controls and the trans.? If you are using a "fits all" plastic tee there may be a port effect. For testing maybe disconnect the whole set up and run the A/C line directly to the intake fitting.