If your pump has not been in use for many, many years the pistons can seize in the barrels. This can make them very difficult to remove without damaging them.
This is the method I use. It works fine and I have used this on many seized pumps when all else failed but you should have tried soaking the pump in carburettor cleaner and a hot ultrasound tank before you resort to this.
First off, make yourself a drift from tool steel. This needs to be to the design shown above. I will post a diagram for this shortly but it’s basically a tool steel rod turned down for the last 41mm to 5.5mm diameter then reduced again to 1.95mm diameter for the last 1.5mm.
The small tip on the end is used as a centre and locates in the hole in the end of the piston.
Now warm up the casting to around 70 degrees centigrade. I do this in my ultrasonic hot bath but a big pan of water on your stove will also do it. Let the heat soak in, then set the pump aside to drain out dry for a moment.
Heat up some penetrating oil in a small pan. Just sufficient to get it good and hot but be careful not to overdo it as we don’t want a fire.
Plug the threads to the fuel gallery with bolts or rubber stoppers and pour the hot oil into the top of the pump filling up the gallery and piston pots.
Now using the drift, carefully tap out the pistons. You need to keep the drift vertical and use several short sharp taps. If you kept the drift upright there should have been no contact with the barrel wall.
Once out the rest of the pump can be stripped down and you can inspect the pistons and barrels.