Sometimes it seems like that what appears to be the simplest task, turns into a major pain in the hind quarters. The plastic screw cap on my 55's stock master cylinder will not budge. I tried penetrating spray but no dice. I despise these plastic caps and not sure what to try next. Appreciate any tips. I know one thing, this cap is going to get replaced by a metal version but first, I have to get this one off! Steve

 

Get a hair dryer and heat the cap enough to hopefully loosen it.

 

You might think the plastic cap stinks, but I actually prefer them. The aluminum versions are prone to galvanic corrosion if they're left in place for a long time and they'll stick even tighter than the plastic ones.

 

Good idea and will give that a try.