Once the site glasses are out, use a oxy-acetylene torch to heat up the threads around the cap. Get it hot and turn the cap with the pipe wrench. I've done this maybe a dozen times. Works every time.

Torch is your friends disassembling a gas pump. Seems dangerous but only once have I had one light up on me (1 of 100 or so). Way faster than waiting for penetrating fluid to work and then that works only some of the time.

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