joe blow track pump problem

I have a Topeak joe blow track pump, I'm having a hard time trying to fit it to a presta value, sometimes it works fine other times it will not pump air into the tyre, the handle is impossible to push down after one pump, any idea what I'm doing wrong? I pull the yellow level to the opposite side to block the air coming out the other side, or is it a faulty pump? as the joe blow seems to get mixed reviews.
(I'm opening the presta value)
(I'm opening the presta value)
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was trying out a new mini pump, must say I was really surprised at how fast it pumped up the tyre, was considering buying co2 for flats but no need now.
This...also I find the amount you "push" the thing on can make a difference, sometimes no amount of pressure will put air into the tyre and I have to undo/redo the thing to make sure its on the right amount. Don't know if I'm pushing it on too far or not enough...I suspect too far.
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I press it down and let a little bit of air out - that way I know it's free, otherwise the valve usually gets stuck and no amount of pressure in the pump will open it (well - no amount I'm prepared to try anyway)
I just thought it was either me not fitting it on the valve correctly or a duff pump.