Reducing Slamming of Two Check Valves on Sump Pump Discharge line

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.

http://www.deanbennett.com/basement-sentry-507-install.pdf
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?


Thanks!

I think you read into the instructions a bit too much. You only need one check valve between the two pumps.
The back-up pump has it's own check valve built into it.

So, when I installed the new check valve between the two pumps I should have removed the original check valve that was above the pit?

Do you know of anyway I can silence the check valve slamming or am I stuck at this point?

Yes.
A silent check valve is a silent check valve.
If you stack 2 check valves, you are asking for trouble.