I have a question about the atmosphere poppet valve in this booster unit.

Can anyone tell me how freely this poppet valve should move?

Inside the piston of this unit:

The pivot arm has the vacuum poppet valve and return spring on one end. On the other end of the pivot arm is the atmosphere poppet along with a hold down spring.

If I press down on the atmosphere poppet end of the pivot arm, the atmosphere poppet moves downward and the hold down spring compresses. When I release the arm, the poppet position remains where it was and the spring remains compressed. What I don't know is if this is normal behavior. I would expect the hold down spring to return the poppet to it's original position. The poppet has a snug fit. It is clean, the tube it rides up and down in is clean.

With the engine idling, I measure 18 inches of vacuum. Step on the brake, the vacuum drops to 10 inches and the engine idle drops and the engine shakes a bit. I am thinking vacuum leak in the booster, but there are no tears in the diaphragm and the vacuum poppet appears to be moving properly. The piston o-ring is good, so I'm wondering about the atmosphere poppet. I removed the vacuum reserve canister from the circuit to eliminate it as a source of leakage as well.

While there will be a momentary drop in the vacuum when applying the brakes, that vacuum should come right back even though the brake pedal continues to be depressed.

Lorena, Texas (South of Waco)

What I'm seeing is the vacuum drop to 10 and remain there while the brake is depressed. You're right, I would expect the vacuum to drop, then come back up. So it appears there is a leak in the booster. I was looking at the atmosphere poppet as the culprit, but it might be something else. Maybe the piston o-ring. This one looks like it's made of rubber. I was expecting leather, just hard to tell what it's made of.

The book diagram show that when the brake is depressed, the vacuum poppet opens and the atmosphere poppet closes.

If I read it right, holding the brake down, both poppets are supposed to be closed, which would then allow the vacuum to build back up.

Another process that may be in play is the bleeding of the brakes. We've only bled AT the master cylinder by cracking the line and stepping on the brake pedal, then closing the line. I saw someone post something about "releasing" the master cylinder during brake bleeding. Not sure what was meant by that. Regardless of the bleeding, that doesn't explain the vacuum loss.

did you solve this treadle vac issue? I have a 56 Mercury Montclair and my power brakes don't seem to work. I have to push very hard on the pedal. I am getting ready to take the treadle vac out and get it apart to try to find the problem. If you solved yours please update on what you found.