If you connect your floor pump to your valve and it pushes the handle back up, the pump's check valve isn't working properly. It's a quick fix for pumps with a metal base. If your pump has a composite matrix base, it's a little more involved.
It's never been a frequent enough issue for us to sell the parts on our site, so you'll have to contact us to get them. If you're in the US you can use the submit a request link in the upper right hand corner of this page. Please include the exact model of your pump and your address. If you're out of your warranty period, we might ask you to pick up shipping.
If your outside of the US, please contact your original place of purchase; they'll be able to help you find a solution.