I also would recommend finding tubes with valves of the correct length. However, if that fails, I've had success using a presta to schrader adapter.  Often with shorter valves, there won't be enough room to attach a pump head, but there will be enough to attach an adapter, which you can then use a pump that works with schrader valves. Most pumps have the ability to do both now. I'm not sure if this will work in your situation, since you say you have 50mm rims and 42mm valves which to me would mean that there would be no part of the valve protruding from the rim.

Non Removable Presta Extension
I had to do a hard power off on my Pi, as it had crashed. Now, when I turn it on, I get 3 flashes of the OK LED, which means either loader.bin or start.elf is not found. I have plugged the SD card into my Windows PC to browse the boot partition and neither of those files is present. Is there any way to repair the boot partition and allow the Pi to boot without losing any data on the card? I have important data that hasn't been backed up yet on the card.
