Personally, if I were REALY having problems getting a cap off to only check the fluid, I might buy a replacement reproduction one and a REBUILD KIT to have on hand, avoiding any down time first (rather cheap and easy to get)
Then, as easy as these Master Cylinders come off of the car, I would probably at this point of having no success trying any methods mentioned above, bring it out and mount it in a vice. Obviously, it has been a while since anyones been into it, right? Probably filthy inside too. I mentioned having a rebuild kit handy (may be seaping some at the push rod anyways.) You might not need it, but it also is cheap insurance. No down time this way. You can always hunt an original cap if you end up destroying it, but with the cylinder OFF, you can really clean up that master cylinder threads, and the whole darn thing while you are there.
Now with cylinder in the vice... on getting the cap off. If HEAT is the answer (maybe so) a hair dryer probably won't be hot enough. Find a heat gun. Gets everything hotter much faster and would possibly expand the metal quickly enough to release it using a quality band clamp or filter wrench as mentioned earlier.
I have another Idea I would try with the cylinder in a vice, I wouldn't try with it mounted in the car. Using the same methods earlier mentioned, or possibly any combination of those methods...excepting the channel locks, that sounds sure to destroy.
While holding an "unscrewing pressure" on say the filter wrench, by using the vibrating effect of an air chisel with a flat or dull chisel end on the hinge or other flat area of whatever tool combination, might shake it loose. Key is not damaging anything here, to also be careful not damaging your tools too. It is tricky, but living in the rust belt, we have to have tricks like these. It also might require more than one set of hands or trying different combinations, but SOMETHING can and most likely WILL work without hurting the original cap.
Is it by chance "REVERSE THREAD"? I've seen that before, but I don't have an early spin-on Mustang brake reservoir and don't remember.
Richard
