Vacuum pump curious behaviour.

I picked up a freecycled vacuum pump and motor a few years back for a round-2-it project for a vacuum bagging clamp to apply veneer to wood.
I'm starting to put the system together for its first job.
The pump is an Edwards Speedivac ED75 run by a 1/3 hp motor at about 500 rpm. Fresh Edwards oil filled just above the minimum mark.
Using about 600mm of 6mm od nylon pipe the output of the pump goes via a non return valve to a 3.9Kg ex calor gas bottle (empty!) with its original valve - volume about 10 litres. This line has a gauge scaled 0-760mm Hg connected via a tee piece.
All seems to work well. There is a minor leak somewhere in the push fit piping but I can live with that.
What I observe and do not understand is the following. With pump running and the valve to the gas cylinder open, the gauge reads 750mm Hg but when I close the valve it drops to 730mm Hg? This is repeatable under steady conditions after the pump has been running for 15 min or so and the body of the pump off cold. I've returned everything to atmospheric pressure and pumped down again with the same result.
The gas ballast valve is closed for all the readings. I can appreciate the gauge might not be accurate but why should the readings be different. I ask from curiosity alone as the level of vacuum I'm achieving will be more than enough for the job.
Can anyone explain please?
TIA
Bob
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On 06/24/17 17:04, Bob Minchin wrote:

So that's a pump, t piece to cylinder and tooling ?. The only thing I can think of is that the cylinder is acting as a filter, smoothing out small variations in the vacuum as the pump rotates. It's quite common on some instrument test setups to put an empty tank in the line for just that purpose. That then affects the gauge reading.
I don't look in here much either these days, seems pretty quite quiet and there are loads of other forums online with more activity...
Chris
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On 24/06/2017 18:04, Bob Minchin wrote:

If I've understood your set up, your small leak is causing this.
With the valve open, the leak has a large volume to reduce the vacuum in. With it closed, the volume is much smaller and you see the impact more quickly.
Not that it is a surprise but it does tell you the leak isn't in the tank ;-)
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Bob Minchin wrote:

Thanks for the replies. It turns out it is the cylinder action as a "smoothing capacitor" in the system. I put the gauge on the end of 30m of tubing and the anomaly went away as the tube volume acted as the smoothing element.
Bob
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