Master cylinder seized

Hi I have master cylinder with a very seized piston It has been sitting a long time with visible rust etc I have read the threads but meed some advice on how to free it up. I will be resleeving after with a 13 mm kit Thx Drummer 99

I had one 2 weeks ago, first I got rid of the rubber boot and clip so the plunger was exposed, stuck it in my ultrasonic cleaner for 40 mins, then sprayed with WD40 and then left it overnight. Next day with some effort the plunger moved in so stopped after 1mm and sprayed with WD40 again, left for another 24 hrs. Then with the master cylinder held in a vice with soft jaws I blocked off one outlet with a dead brake switch (any 3/8 unf bolt or similar will do) and using an oil pump pushed oil into the 2nd outlet and the plunger moved back out until the primary cup exposed the hole into the reservoir. I then pushed the plunger in a bit more and applied the oil gun again. After repeating the push and inject a few more times the whole assembly pulled out with some long nosed pliers.

Thanks for the feedback I have it soaking in penetrating oil What is an oil gun do you mean grease gun?

No, metal oil can with metal pipe and metal rounded nozzle on end.

Ok got it Thx