Getting Plastic Cap Off Valve on Expansion Vessel

[ode]

I need to access the Schrader valve on my expansion vessel but the plastic cap covering it won't seem to come off.

It spins freely and I can't pull it off because I can really get no purchase on the smooth plastic.

I tried squeezing it either side to enable me to get my nail underneath and pull but it's not enough space.

The only option left seems pliers but I'm concerned about damaging the valve.

Any tips you can give me?

Thanks for your help.

 

[polesapart]

It's just a dust cap, no different to bicycle tyre . Pliers are fine, if need be.
Just don't chew up the metal beneath.