On Wed, 04 Oct 2017 19:23:40 +0100, john.west wrote:

In trying to deal a death blow to Japanese Knotweed in the garden, i
have bought an old paraffin blow torch.

Which is of no use at all. Glyphosate is the only thing that will shift
it and is ideally injected into the growing stems. It will takes several
years to completely eradicate it. If you dont wish to follow the
injection route then it can be sprayed, again repeatedly over several
years.

Japanese Knotweed has a number of 'prohibitions' around it and I
recommend that you look them up and check just what you can/can't do with
it (including the soil its growing in)

The leather washer of the pump is very dried out and no longer a good
seal.
To moisten it, to expand the piston leather; am i better using paraffin
or dubbin or would it be kinder to the leather to use a vegetable oil
like walnut oil? Thanks.

If the washer has dried to the extent you suggest then its probable that
only a new washer will be the cure. In the interim you could try a good
soak in paraffin to loosen it a little followed by an even longer soak in
engine oil - if you have any used engine oil then you could skip the
paraffin soak and just try that.