looking for a P337 Rear vacuum relief valve

[weebyf15]

Hello guys, I have a 79 P337 that is not holding suction on the aft engine. On start up its fine but decays away after a few minutes of running. I suspect its the relief valve as the pump has been replaced. I was thinking someone may have removed all their vacuum gauges and may have the old aft relief valve they don't need. You can PM me or let me know if you have any ideas you wish to share.
Thanks Brad