I just had to change the external check valve on the outside of the can on the Pantera. It was stuck open.
Also on my 68 Shelby the original power brake booster finally failed.
I have it apart on the bench now and one of the diaphrams had dried out and split open.
Both of these items will cause issues of low vacuum.
The defective brake diaphram will make the engine vacuum drop to about 6 and not recover until you release the brake pedal. Sometimes it can make the car stall.
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