Not Holding Vacuum On Vacuum Brake

[Tony, NY]

Decided to test my crab distr vacuum brake. Hooked a hand vacuum pump up to the front cover fitting, pumped away and got no reading on the gauge. Removed distr and greased up the gasket and rubber seal. Tried it again and could pump up the vacuum but would drop off immediately. Took out the VB piston and it looks like only a grease coating on the piston is what holds the vacuum. Question is, should that grease coating hold a steady vacuum? What type of grease should be used on it? Thanks.

[Charlie ny]

Tony,
If the piston actuates with steady vacuum you're good, I do not believe
the piston could hold vacuum even when new let alone after 65 years of use.
I test my distributors with an electric vac pump which is capable of 25
inches of steady vac. When the vac hose is pulled off the spring pushes the piston
down pronto. I leave the piston and its bore dry so as to not attract contaminates.
Charlie ny

[Old Henry]

The vacuum brake is not supposed to "hold" vacuum. It is activated to varying degrees by varying vacuum. As the vacuum decreases it is suppose to drop down onto the timing advance disk to "brake" it and retard vacuum. As the vacuum increases it lifts the brake off of the disk to let the mechanical timing advance work to advance the timing.

Here is how the vacuum advance brake is adjusted:

[supereal]

With today's gas, finding a "ping" is unlikely. When we set up a distributor on the machine we back off the adjustment bolt all the way. If you find a deep groove in the piston face, it is likely dragging all the time, affecting advance.

[Brian]

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The vacuum brake is not supposed to "hold" vacuum.

So...if it is unable to 'hold' vacuum, obviously this would be the source of a vacuum leak, albiet, a small one. I always, [unlike Charlie] apply a lite coating of moly grease to mine upon assembly in order to assist the 'seal'. Note, there are two different housings too, an aluminum one, and a cast iron unit, which has a bronze bush in it and is my personal preference because of the bush.
I, like Supereal, back the bolt right off, this allows the unit to advance, whilst still imparting a slight drag to the unit, thus preventing an erratic, jerky movement as could happen if one completely removed the brake. My opinion only.