A ball bearing as a non return valve and a piston with holes and a round T shaped valve that sits inside the piston
Dennis, I am interested in pictures of the piston of the pump. Actually of the whole inner workings. I have followed your report on the construction of the Pericaud, but I have not seen detailed pictures of the piston and the non-return device. Or did something pass me by? My old head is a bit sloppy sometimes. 
The non return valve is a ball bearing at the bottom 0f the column that rest in a bevel edged hole in the base .
The piston is hollow with 5 holes in the head of it.
1 is in the center and 4 around that .
A T shaped piece .( the leg is a loose fit in the center hole .
The top of the T is a disc that covers the other four holes .)
The T drops in to the piston and moves up when the piston goes down thus allowing water through
The piston and around the piston rod and when the piston comes up the T valve (seals?? ) the four holes .
The piston rod connects to the piston via a wrist pin allowing movement .
I will chicken scratch some pictures later