Old style 880 Chamber Seal/Pump System

[Ronno6]

The original 880 chamber seal was a floating seal that was held against an internal ring swaged into the pump tube by a spring.
The seal backup disc features a convex rim that nests onto that ring.
That is what holds it in place from moving forward up the tube.
The piston had a similar convex rim that nests onto that ring from the opposite side.
The faces of both the seal disc and piston protrude into that ring, and they can actually contact each other, with a bit left over.
That is to say, the piston face will contact the seal disc face prior to bottoming on the ring in the pump tube.
This allows for some movement of the seal disc....

Now for the fluid dynamics of the system.......
The pump must increase air pressure in the pump tube to a pressure greater than that in the chamber in order to force air from the pump side to move past the seal into the chamber. The chamber seal acts as a check valve to contain that pressure.
A pressure differential is required on every pump stroke to move air past the seal into the chamber, raising the PSI inside the chamber.
With each stroke, the pressure in the chamber increases, and air does not enter the chamber til closer to the end of each pump stroke when the pressure increases sufficiently in order to pass the seal.

Now, things can get interesting......
Because the chamber seal can float by means of compressing the spring and air in the chamber, it is possible that not all the air
being pumped will pass the seal and enter the chamber.
However, if the seal DOES compress a tad, it INCREASES the PSI inside the chamber by reducing the chamber volume.
It is possible that, once the exhaust valve opens and dumps the air in the chamber, the residual air on the piston side of the seal will
blow past the chamber seal....the whole system acts as the chamber.........

My brain hurts... (fingers,too!)

[caudie]

A lot of difference between the old and new.  I didn't comprehend all of that but it answers a lot of what I was trying to figure out between the 2.  Thanks for that.  I know there's a lot more differences also.  Like the metal vs plastic pump handles.  I think, if I'm not mistaken, there were 2 metal handles and 2 plastic handles to add in.  Over the 52 years of 880's lots of changes.

[Ronno6]

There were 3 or 4 different pump handles; 2 or 3 were formed sheet metal w/cast handled and different latching systems;
Only 1 all cast metal handle.....

[Back_Roads]

 "It is possible that, once the exhaust valve opens and dumps the air in the chamber, the residual air on the piston side of the seal will
blow past the chamber seal....the whole system acts as the chamber........."

 I believe it does, as proof the pump handle jumps for lack of a better word, on firing, indicating a release of pressure from the piston side.