I have a cheap Chinese pump that worked pretty well until recently. The problem is that something happened and the pump doesn't work so well anymore. It's hard for me to explain exactly the issue but when I start pumping I feel tension when I push but I feel even more tension when I pull, respectively the pump stays in the middle position and doesn't move either up or down. I opened it, at first glance there was nothing wrong but I still replaced almost all the o-rings as I cleaned and reassembled it. The problem remained as before. Can anyone help with ideas?