F60L master cylinder plastic cap

[Al.Lyon]

I have a 1945 Ford F60L.

It has a newish replacement stainless steel brake master cylinder with a plastic fill cap. The brakes work very well.

(The part looks exactly like the one I’ve attached).

Only I’ve over tightened the plastic cap and I can’t unscrew it. I’ve already damaged the side of the cap by trying to wedge it open with a chisel and a hammer...yes, I’m ham fisted.

What’s the way forward? Will it come off and I just buy a replacement cap? Where do I get a cap? How else can I get the cap off? Will I have to remove the entire master cylinder and then bleed the brakes too?