Practice safety with air or grease
I admire Richard Prince (the Mechanic on Duty) for his knowledge of all aspects of car restoration.
There are very few times when I disagreed with his advice and I never had a major disagreement until now.
There are two possibilities if you do this. The first is that the 100 psi air pressure is not sufficient to push the piston, which remains stuck. The second is that the piston moves and becomes a projectile! As it is likely that the restorer looks in the bore to see what happens, you might end up with a “one-eyed” reader. This is if the reader is lucky... Therefore, issue a correction in bold letters. And replace “high pressure air” with “use a grease gun.” Grease guns can easily develop 1000 psi, and the grease moves slowly. Here is even better advice: Throw away the master cylinder and buy a new one. They cost no more than a rebuilding kit and they save you a lot of work and frustration.
Thank you for your kind words, and for taking the time to write in and share your thoughts. You are quite correct when you point out that freeing a stuck piston does pose a potential safety hazard, but this is true regardless of where the force applied to the piston comes from.
So regardless of what method you choose to free any stuck part, it is essential to make sure that it won’t impact anyone or anything of value if and when it breaks free. It is also essential to always wear proper eye protection when using any technique in an effort to free anything that’s stuck.
Your point about not messing around with a stuck piston and buying a new master cylinder instead is well taken but that is not always possible because new masters are not available for every collectible vehicle out there. Also, some people are determined to retain their original master and, therefore, won’t consider a new replacement.
And, finally, in many instances it is not true that a new master cylinder costs no more than a rebuilding kit and the difference can be more than enough to motivate some people to fix what they’ve got rather than buy a new part.