I picked up a seemingly decent Trek floor pump at a thrift store for a couple bucks. The pump head is one that "locks" onto the valve (in my case presta) by flipping the lever on the back of the pump head. When I get up to around 100psi while inflating tires, the pump head just pops off the tire valve. I'm assuming it simply cant hold onto the valve tight enough. Can I replace the pump head? Silca heads seem easy to come by and I thought about putting one on there, but wasnt sure if it would fit or work.
Also, I've never had this problem when I use a silca pump and they have no locking levers or anything. How do silca heads stay on the tire valve at high pressures? Thanks!
There may be a bit of residual oil on the head or on the stem. Wipe the stem down with some alcohol and swab the rubber on the inside of the head with a q-tip and alcohol.
I used to have this problem with a serfas pump head that would slide up until it caught the threading for the cap. The alcohol solved it.
Most of these have a screw-on cap to hold the rubber "gripper" inside. If you screw it on a bit tighter, it will compress the rubber seal a bit and make for a better grip.
Usually, the company that makes the pump will sell you replacement rubber bits as well; Blackburn sent me freebies for mine as they had quit making that model.
I just bought a replacement head for my Ozone floor pump at the LBS. It cost me $6 and works perfectly. I had the same problem you described.