Andrew just something to consider. there is a rudimentary seal under the pump diaphragm. Can I assume you have not checked it. The seal supposedly stops oil getting through to the under side of the diaphragm from where it exits at the breather hole (in the bottomside of the pump casting. It has to have the hole. I do think your main problem is not here though.
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Bluebell
Carrier Armoured O.P. No1 Mk3 W. T84991
Carrier Bren No2.Mk.I. NewZealand Railways. NZR.6.
Dodge WC55. 37mm Gun Motor Carriage M6
Jeep Mb #135668
So many questions....
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