I recently finished making a replacement piston for the Presso. The original hung on for about 5 months of daily use, then started developing several cracks.
The replacement I made works, but has two problems.
The first is that I cut the bottom of the piston to fit perfectly into the bottom of the piston-chamber, but if the alignment is off by a little bit the piston does not go all the way down. I would suggest to anybody making one that they simply lathe the bottom so that it fills most
of the space, this way it is not likely to get stuck.
The second problem is that the diameter of the cylinder of PFTE that I was working is a tiny bit smaller then that of the original piston. What this means is that it is possible for the round 'washer' to be forced out of it's slot, sending a gush of water all the way up to the ceiling . . .
For the next one I will definitely use a lathe. I tried sticking it on the end of a power drill and this resulted in awful alignment. I kinda lucked out in that if I turn it in a certain direction the 'washer' does not get spit out while under pressure.
Here's a picture-