Thanks to the experts on this forum I was able to disassemble my Parker 51 without inflicting any damage. Hot water (175 degrees), two short pieces of flexable rubber fuel line and some gentle torque applied by two pair if slip-joint pliers did the trick. A through flushing with warm water and careful reassembly and the pen is as good as new. Fortunately the breather tube was intact. Thanks again the this forum, my first attenpt at pen repair was successful.

Congratulations, still plucking up the courage to try this, but I have a rather battered speciman, that I may experiment on shortly, as it doesn't want to write well at te moment anyway.