Got in the workshop today and roughed out a few bits.
I expected to have a couple of attempts at making the piston but it seems to have worked first time. The design is copied from the pistons on a couple of mini pumps I have, a clearance fit piston with a tight fitting O ring in a 'cut away' slot. The principle is that the O ring seals on the push stroke but is not supported fully (by the cut away slot) on the pull stroke and deforms to let air past. I just used the nearest O ring size I had and winged it with all the piston and groove dimensions so I was quite pleased when it worked first time.
I am waiting for delivery of some umbrella valves before I can start on the business end of the pump.
For simplicity on the first one I might connect to the inner tube valve with a flex hose donated from another pump, this just needs a threaded hole in the end of the pump. I would like to do away with the hose and connect the pump direct to the valve but haven't yet figured how to make and fit a reliable sealing arrangement in the limited space available.
Hope you are finding this interesting - I am
For the weight weenies, weight so far 84 grammes
