By Pass Valve:
Blow off valve, dump valve, same thing.
Here is the replacement of the OEM (upgraded) Bosch NP710.
NOTE VALVE ORIENTATION. Valve must go ONE way.
The little clamp is a squeeze type. Difficult.
The crimper tool is identical to a irrigation installation clamp tool.
The top of the valve is fed by vacuum through the braided hose.
Its the black sideways canister, dead center of the photos above & below.
There are two clamps, slotted for screw drivers.
We used a 1/4" socket wrench.
Top clamp above, bottom clamp below.
NOTE VALVE ORIENTATION.
Valve must go ONE way.
To test old valve apply or make vacuum to the "inlet" end.
If air leaks by (vacuum is lost) valve is bad. Possible to clean.
Valve must seal to work correctly. Part does not disassemble
Swap out old one and put in new one.
Pressure testing By Pass Valves: (BPV)
Here Art and Chester and I check some Bosch NP710 by pass valves for leakage.
Here is the (spare) Turbo hose with the BPV oriented (right)
Chester's cool pressure gizmo below:
New Bosch valves held vacuum until 7-8 psi, old used ones leaked at 4-5 psi.
Other by Pass / Blow off valves (Diverters) are tested towards 15-20 PSI.
They have a piston valve and stiff spring.
The Bosch valves have a diaphragm.
Here's a link to an excellent BOV / BPV test:
http://wwwrsphysse.anu.edu.au/~amh110/Technical_pages/blow_off_valve.htm