[AllenG (OP)]

How would one go about modifying a bicycle pump so you can use it to make a vacuum?

I want to make a few small composite parts using a vacuum bag and a hand pump would be handy.

[Don_1]

Try inverting the rubber cup washer on the end of the plunger and a reversed valve on the end of the connector. I doubt it would be very efficient, but should give a reasonable result..... I think!!! []

[lyner]

It depends what pressure you need. You would need another  valve on the inlet to the pump if you want to pump it more than once (a tyre valve could probably be adapted easily).

Did you consider using a vacuum cleaner? They're very handy if you can make do with a relatively small pressure difference.