On Tue, 06 Nov 2012 20:48:11 -0800, DD_BobK wrote:

For OP & anyone else who to learn about backflow preventers I offer...
http://content.zurn.com/web_document...on/ISSM950.pdf

That document describes my backflow preventer:
"Zurn Wilkins 1-inch ID Model 975XL Reduced Pressure Principle
Backflow Preventers are for use on potable water lines where a
health hazard could exist if a backflow situation were to occur."

And, that document describes the four test cocks:

1. Start with both shut-off valves closed.
Slowly open the inlet shut-off valve until the backflow preventer is
completely pressurized. A brief discharge from the relief valve may occur
while the device is pressurizing. The discharge should cease by the time
the shut-off valve is fully open. Device should function properly. If the
discharge does not stop, refer to "MAINTENANCE INSTRUCTIONS" for repair
procedures.

2. After the device has been pressurized, vent all trapped air from both
check valve by slightly opening each of the four test cocks.

3. Slowly open the downstream shut-off valve. The Model 975XL Reduced
Pressure Principle Backflow Preventer is now in service.

4. If "spitting" or intermittent discharges from the relief valve are
noted, it could be a result of pressure fluctuation and/or a water hammer
condition in the system. If such conditions exist, install water pressure
reducing valves or water hammer shock arresters in compliance with
industry standards as needed.

5. After the Model 975XL has been properly installed, test the device
(see "TEST PROCEDURES"). If the device fails the test, remove the first
and second check valves and thoroughly flush the device. If the relief
valve fails to operate properly,inspect the sensing passage for clogging
(see "MAINTENANCE INSTRUCTIONS"). Clean rubber seals of all debris and
place unit back in service.

The TROUBLESHOOTING section is apropos for a.h.r.

TROUBLESHOOTING THE MODEL 975XL:
When the relief valve discharges intermittently it can be almost always
assumed that the device is functioning correctly and that the
discharge is caused by systems such as inlet pressure fluctuations or
water hammer due to quick closing valves.

The amount of discharge is proportional to degree of fouling. Most
problems occur in the #1 check which is where debris enters the backflow
preventer first.

1. SUDDEN OR RAPID SPITTING
a. Drop in inlet pressure.
b. Sudden increase in downstream pressure due to water hammer from quick
closing shut-off valve installed downstream.
A. Install an in-line spring loaded check valve upstream of backflow.
B. Install pressure reducing valve upstream of backflow unit.
C. Install in-line spring loaded check valve downstream of backflow as
close to source as possible, but not closer that 4 feet.

2. LIGHT INTERMITTENT DRIP
a. Slightly fouled #1 check.
A. Clean #1 check and turn check valve seal ring over or replace.

NOTE: Continuous discharge of the relief valve signifies a failure of
some part of the device. To help determine the specific area of failure,
close the #2 shut-off valve. If the discharge stops, the #2 check
requires service. If the discharge continues, the #1 check requires
service.

1. CONTINUOUS DISCHARGE