I can't pump my threadless presta valve. It just slips off everytime. What bike pump do I need?

Just buy a presta to schrader adapter,that is if your pump will work with a schrader valve.I had the same problem my valves weren’t long enough

don't those adapters screw on?

I have a Bontrager floor pump and have no problem with the smooth Zipp valve extenders.

Yes they screw on but to the threads that the valve stem cap screws on too.Im using valve extenders on my new wheels though.

A new tube is cheaper than a pump

Ditto

The washers in the pump head can wear out and not hold a smooth stem. Buy tubes with threaded stems or a replacement gasket, head or pump, depending. Some pump brands will sell or give you replacement washers.

I've had to slide & affix heat shrink over the smooth shank for a pump head to clamp tight enough.

When I had this problem I just changed the pump head. Now I have the opposite problem: the new head is so tight it's somewhat hard to get on and off. But it always seals and stays on when pumping. Maybe I'll try putting a little silicone lubricant on it.

^^^ Little bit of wisdom right here! My Park floor pump constantly has issues keeping a tight seal on any Presta valve... I'll have to try this as it's a cheap and easy hack.

PoorInRichfield I've tried RTV [black] , but it wasn't consistent for smucking it on smooth.

Not by much

And if the current pump isn't working well now that's a good reason to replace it