Zéfal Air Pump gauge not working

I've had something similar on my Topeak Road Morph; in fact you remind me I need to strip the gauge again. My pump stays on the frame in all weathers. Maybe yours does too and suffers in the same way.

What I've found is that the gauge can get muddy water in it, with enough grit to make the moving parts stick, either completely or partially (the latter cost me a tyre when I overinflated it and blew out the sidewall). It wasn't seals that were the problem, but the spring and sliding indicator. A good rinse and re-grease did the job for a year or so.

Even if you can't fix it and the gauge is in the head, you may be able to replace the entire head, either with an identical one (probably the best idea for portable pumps as you don't want it flapping around) or a similar one (on a track pump). On my track pump the gauge isn't on the head but on the pump itself - it does however use a standard pressure-fitting thread so any pressure gauge of the right range would do.

This is an interesting question. Without the pump in hand, only somewhat informed speculation is possible.

All pumps have o-rings inside to create seals. O-rings are made of rubber, which can degrade with age - basically, it becomes less elastic, and maybe cracks. If your pump weren’t pumping entirely, there's definitely an o-ring inside the main chamber that's attached to the piston. This probably would need replacement.

If your pump is merely not reading pressure as you described, there may be some o-rings controlling the flow of air to the pressure gauge that have degraded. I couldn’t find instruction videos on Zefal’s site, but this video from Lezyne may be instructive. The presenter is replacing the pressure gauge. At about 1:58, he references an o-ring that seals off the pipe to the pressure gauge. Alternatively, it's possible that your gauge itself is broken.

Zéfal does say they stock replacement parts for all their pumps, but it wasn’t obvious which o-rings you’d need to look at on their website. You could ask which o-rings apply to the pressure gauge, and if they can be replaced without removing the gauge itself. Alternatively, it might be worth asking at a local Zéfal dealer, since the company is French; I don’t know that there will be language barriers, but if you don’t know exactly which model you have or exactly what is wrong, it may help to converse in a language you and the other party both speak at a native level. I wasn't able to see spare gauges on Zefal's site, but their text about committing to product support implied that they stock the gauges.

In any case, in principle, many floor pumps can be repaired, but people will probably weigh the cost of repair, the potential effort, the probability that they mis-diagnosed the issue at hand, and the cost and potential features of a new pump. I do encourage you to try to repair this one, but it’s possible that I am wrong about which part has failed, or that you haven’t described the issue adequately, or something else.