I installed a Zoeller Basement Sentry 507 battery backup pump inline with the discharge line as shown in the installation manual below. It required me to add a second check valve on the discharge line between the main pump and the backup pump.

Page 5, figure 1..

I then replaced the original check valve that was located about three feet above the basement floor with a Klunkless Basement Watchdog check valve to relieve the check valve slamming that was happening after the pump shut off, but this has not reduced the slamming noise.

I guess I have a few questions:

1. Will a "klunkless" valve stop only hammering or will it also close quietly and not "slam"?

2. Is it possible that a "slient" valve can be rendered useless when there is a normal check valve below it? It's possible for the lower check valve to still slam shut if the upper silent valve is open a little longer duration of time?

3. Is my only option at this point to replace the Watchdog valve with a Campbell quiet valve and see if it improves or is the physics of question 2 impossible to overcome?