Dribbling Injection Pump on my Fordson Dexta

My 1960(ish) Fordson Dexta has started to drip diesel out the the breather pipe coming out the bottom of the injection pump. The pump is the older Simms pneumatic version which is regulated by the diaphram. I don't think the diaphram is perforated because the engine revs are stable (ie it does not "hunt"). Has anyone else seen this problem, or have a remedy.

Many Thanks
Ian.

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The problem has been getting worse recently and at worst can mean a constant stream of diesel running out the pipe, I've taken the inspection hatch off the side of the injection pump and I can see a small amount of diesel at the bottom of the pistons when the engine is running, but this might be lubrication diesel. The problem is worse when the tractor is running at low revs. I've also noticed that if I manually pump diesel using the fuel pump that diesel will run out the pipe. I'm wondering if the membrane in the fuel pump is perforated and is allowing fuel to drain back into the injection pump and down through the pipe at the bottom. This would account for the low revs causing a greater flow as the pressure at the fuel pump will be higher. It would also explain why the fuel runs out the pipe when I manually pump the fuel. Is this feasible or is it just wishful thinking?

Thanks Again
Ian.