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.