2 minutes ago, Captain Obvious said:

I think as long as it depletes a set amount, the diaphragm is intact. The "set amount" thing is promising.  From what I can tell (from a distance and all that) it sounds like a good booster.

How much vacuum are you able to draw down on it? Hand pump, or you got something powered?

I'm using a hand pump and I've gotten it up to as high as 22lbs before my hand said enough.

It's ok its depeleteing a set amount even though vacuum amounts are being detracted on pedal push in & the same when the pedal is returned (the gauge drops pounds on 2 different actions)?

There should be some decrease in the vacuum on pedal push, but I would expect there to be more on pedal release. As long as it doesn't continue to bleed out when you hold the pedal push constant, then I don't think you have a leak in the booster diaphragm.  I'm no booster expert and I hope I'm not coming off as one, but from my looking at the design, I believe that's the case.

The whole thing is actuated by a pair of valves on the pedal shaft. With the pedal not pressed, it opens a valve between the two sides of the diaphragm and allows vacuum to build and equalize on both sides. Then as you press the pedal, the first thing that happens is it closes off that valve between the two sides. Then as you continue to press further, it'll open up a second valve to vent off some of the vacuum on the rear side.

What I'm trying to test with the "set amount" of vacuum bleed on pedal push is... Does that "close before open valve" work properly and are there no other parallel paths between the front and rear sides of the diaphragm (like a hole).

I'm trying to come up with scenarios that you can look for on the bench that give you some confidence in the operation.

"Holds vacuum indefinitely while static" is a good first start.
"Goes whoosh when you press the pedal" is a good second test.
"Drops a set amount, but not all the way when you press the pedal" is a good third test.

if it passes all three of those I expect it's good enough be worth the time to install it and give it a try.

On 6/3/2020 at 8:55 AM, Captain Obvious said:

There should be some decrease in the vacuum on pedal push, but I would expect there to be more on pedal release. As long as it doesn't continue to bleed out when you hold the pedal push constant, then I don't think you have a leak in the booster diaphragm.  I'm no booster expert and I hope I'm not coming off as one, but from my looking at the design, I believe that's the case.

The whole thing is actuated by a pair of valves on the pedal shaft. With the pedal not pressed, it opens a valve between the two sides of the diaphragm and allows vacuum to build and equalize on both sides. Then as you press the pedal, the first thing that happens is it closes off that valve between the two sides. Then as you continue to press further, it'll open up a second valve to vent off some of the vacuum on the rear side.

What I'm trying to test with the "set amount" of vacuum bleed on pedal push is... Does that "close before open valve" work properly and are there no other parallel paths between the front and rear sides of the diaphragm (like a hole).

I'm trying to come up with scenarios that you can look for on the bench that give you some confidence in the operation.

"Holds vacuum indefinitely while static" is a good first start.
"Goes whoosh when you press the pedal" is a good second test.
"Drops a set amount, but not all the way when you press the pedal" is a good third test.

if it passes all three of those I expect it's good enough be worth the time to install it and give it a try.

Gotcha, thanks so much for your help.  I'm guessing that the small increase in engine rpm when the brake pedal is held is probably coming from the stretched/hardened ends of the brake booster hoses.  I clipped the ends and re-installed them.  I will test the car in the next few days.

Well if you're hitting the brakes and the RPM's are going up, then that's most often an indication of a leak somewhere. And most often that is a hole in the booster diaphragm.

So are you sure that when you actuate the booster and hold the pedal position constant, it doesn't continuously bleed off your vacuum on the bench?

It won't fully deplete the booster's vacuum if the pedal is held.  The gauge will drop about 5lb of vacuum on pedal-in and 5lb on pedal-out regardless of how long I hold the pedal down.

The interesting bit is there is a hiss sound on pedal-in/pedal-out if the vacuum gauge is below 4lbs of vacuum remaining in the booster.

Update: The source of the vacuum leak (where the engine rpm would rise when the brake pedal was pressed) was the ends of the brake booster/check valve vacuum hoses.  They were old/flayed so I snipped the ends off and put new hose clamps on em - worked like a charm.

Thanks for your help, everyone!

Awesome. So while you were in there messing around, did you peek inside the brake booster and verify that it is clean and dry? It hasn't sucked in any brake fluid in the past, right?

I didn't open it but there was no fluid in/around the brake booster also that master cylinder is less than a year old so it should be good