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Vacuum Heatercock operation


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I have the dash out and have removed the blower. I thought I would replace the original heater hoses while I was there. To access them, I removed the heater valves. I thought I would check the operation of the vacuum heatercock. The vacuum will retract the cawk, but in its deactivated position, it will not block the flow as it should.

I have attached a view up the exit pipe.


It's hard to see, but the cawk does not seal at all against the pipe seat.  With the pipe removed, you can see that the cawk extension is about 0.92", but the pipe seat is 1.04" deep.



I see that ZCarSource offers to rebuild these, but I managed to break off one of the 3 mounting studs on the vacuum mount, so I don't know if they would accept my core.

I have studied the purpose of this valve, and it is intended to totally block water flow into the heater core in all modes except heat and defrost, thus blocking flow in 'Vent', and 'A/C' modes.  However, in vent-A/C, there is no airflow through the heater core. So, the vacuum heatercock apparently functions only to limit any stray heat which might radiate or conduct from the heater core. Datsun did not think this was necessary in their non-A/C Z's, which would have benefited in the 'vent' mode.

I also checked the heater control valve, which is operated by the 'temp' lever, and it does not seem to totally block water flow at its coolest setting.

I could just re-install the non-functional heatercock (perhaps after leak-testing it), to retain heater/defroster operation, and tolerate the unwanted stray heat in other modes. Or I could just totally block coolant flow into the heater, avoiding the stray heat, and leaks from those valves, hoses, and core. Note that here in Houston, even on my daily drivers (which this will not be), I rarely need heat.

Any thoughts will be appreciated.  Thanks.


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These are fiddly devices and maybe not worth trying to fix by yourself.  And the repaired unit might just reward you for your work by starting to leak instead of stick.  Why not just call ZcarSource and ask them for their price without a core?  

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Thanks, guys.

ZCarSource did confirm from the photos that they cannot rebuild this because I broke off one of the studs. I will just plug the heater lines. Someday, if I get really curious, I will open up the unit to see what exactly failed, causing the limited cawk extension.

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