I believe the shop manual refers to the vacuum dash pot on the throttle relay lever as a check valve. I have finally corrected the other vacuum leaks in the A/C heater system only to find the vacuum unit on the throttle linkage will not hold a vacuum. My question is,  does this diaphragm have a calculated bleed or is it supposed to be vacuum tight? Mine is working with engine vacuum but I can't draw a holding vacuum with my mighty vac hand pump. Thanks.

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It should hold vacuum, and when the engine rpm's drop, the vacuum drops, allowing the plunger to come back out.

I have a good part, just send me an email;  russ85747@hotmail.com

Just to add to the discussion, Cadillac replaced the vacuum fed check valve early during the 1964 model production run, with a spring and orifice type dash pot.  According to the serviceman bulletin they went to the dash pot "to prevent stalling after sudden release of the accelerator pedal." according to the Serviceman Bulletin.  I've got both the vacuum check valve and spring and orifice dash pots.  If you wanted to switch over to the spring and orifice dash pot you would also need the throttle relay bracket it attaches to.  

Thanks guys!

Jason, is upgrading to the 64 design a remove and replace bolt it in, connect the linkage and go task? I like the prospect of eliminating a future vacuum leak if it is R&R then go. Thanks.

David 

Glad you found the vacuum valve not working correctly and asked about it 

You can just plug any vacuum line you may find leaking until you get to the repair 

It helps the rest of the system to perform as intended 

As the intake manifold is the source of all vacuum any vacuum leak in the cabin or engine can effect the engines performance directly to lean the engines fuel mixture 

Check your vacuum advance for holding vacuum if by chance you have not 

Enjoy 

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