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According to the manufacturers instructions.
We do not know what you mean by "PFD", please do not use abbreviations. But the answer would be when the instructions for the "PFD" say it needs to be replaced.
For the newer Inflatable PFDs, most manufactures will tell you when you should replace your cylinder, and most have an easily viewable inflator status. For instance - The Mustang Deluxe Automatic Inflatable PFD: Universal fit states: No inflator maintenance is require for 5 years unless the PFD has been inflated. You can easily view the inflator status and replacement date through the safety inspection window. Green indicates ready for use. However, each manufacturer can have different specification, so it's best to check with your manufacturer and for your specific vest type. For more information regarding the Inflatables, visit: The link below
what sort of cylinder you want to replace?
you just replace it
Where's the leak? Wheel cylinder, repair or replace, rusted line, replace, master cylinder, repair or replace.
How I can replace ignition switch cylinder from Isuzu Rodeo?.
How do you replace a slave cylinder in a 96 chevey k1500
For the newer Inflatable PFDs, most manufactures will tell you when you should replace your cylinder, and most have an easily viewable inflator status. For instance - The Mustang Deluxe Automatic Inflatable PFD: Universal fit states: No inflator maintenance is require for 5 years unless the PFD has been inflated. You can easily view the inflator status and replacement date through the safety inspection window. Green indicates ready for use. However, each manufacturer can have different specification, so it's best to check with your manufacturer and for your specific vest type. For more information regarding the Inflatables, visit: The link below
yep, though you may have to replace the mounts.
Ho do you replace a master cylinder in a 1989 Winnebago Superchief?
Possibly, sometimes the brake cylinder develops rust and sticks. If a cylinder has problems I find it easier to replace the whole cylinder or caliper.