I have bought new brake slave cylinder plungers. The cylinder pin seem very tight going into the plungers. When I blew the cylinder out again the back of the piston come out of the piston and stayed on the pin. What is this valve (I think) for and should it be tight going on that pin

 

Are you talking about the brake caliper out on the wheels ?
If the pin came out with the piston, I don't think it can be put back in, it's crimped in there from the factory, or at least mine are.

The return pin in the caliper is tight, but not so tight that it should pull out.
Did you lubricate the bits with brake fluid first before assembly ?

Where did you buy the new pistons ?

 

The pistons came from Jag. The pins are locked in the caliper housing and go into the pistons. Was very hard to push pistons into caliper. Lubricated with rubber grease first. When I put shop air into caliper to remove the plunger, the guts of the plunger stayed on the pin. Wonder if the plunger has a smaller hole than the older ones. once the guts came out of the plunger it is like a spring that goes around the pin that is inside a sleave that is then swayed into the plunger. Any idea what it is for ,(Maybe a dampner)

 

The design (and the only one like it) pulls the piston/brake pad slightly of the rotor (disc) when you take your foot of the pedal.
That what the spring is doing in there that's wrapped around the pin.
They are tight, I use a couple of screw drivers to leaver the piston out.
When assembling I use the brake fluid to lubricate things and squeeze with both hands to push the piston (plunger) in place.

 

Thankyou Jeff. Wasn't sure if it was meant to be like that or not. Happy man now, I can put it all together and know it is correct