pump not pulling vacuum

[theisleof]

Hopefully this is ok to post in a gas chromatography forum. I recently rebuilt an Edwards 3, thought everything was okay, as I had no leftover parts and it started up and ran overnight.
But now I'm realizing it isn't providing much vacuum (MS says it's 4000 mtorr or more), and there is a ton of air exiting out the exhaust port even when left for 20 minutes. It does seem to be providing some vacuum, as it pulls in the sideplate some. But when I switched pumps on that MS, it immediately pumped down normally.
Obviously it is something I did wrong when rebuilding, but I'm wondering if anyone has any obvious ideas of what to check based on what I am describing. Would be much appreciated. Thanks.

[TCCrab]

I've rebuilt a few of these over the years, places to check will be the external O-Ring on the vacuum hose connector and the O-Ring directly underneath the vacuum fitting on the pump itself. If it's venting air via the exhaust port then you've at least got the stator and the internal O-Rings of the pump correct.
It has to be sucking air from somewhere above the oil level in the pump itself.
I've had the occasion to have disassembled a pump completely, finding nothing obvious wrong with the original rebuild, reassembling it and it work just fine.

Good luck!

'Crabs

[theisleof]

Thanks @TCCrab! I'll try to update, but it may take me a while to get around to getting this done.

[theisleof]

Looks like I had the dump valve backwards! No mass spec available to test it on at the moment but it looks to be pulling good vaccum.
Hope this helps someone in the future.