<VV> Mechanical Fuel Pump reverse flow?

[Dave Morris]

On my airplane's Corvair engine, I am planning to use the mechanical fuel 
pump but also have an electric fuel pump in parallel that I can switch in, 
in case of vapor lock or failure of the mechanical pump.  Does anybody know 
whether the mechanical pump has a mechanism that would act as a check 
valve, preventing the reverse flow of fuel backward through the pump when I 
switch the electric one on?

Thanks,
Dave Morris