Have a 1964 B Body that the brake booster Midland Ross check valve might be bad.
How do you test and were are the replacements found.

Thanks

fast check, remove it from the booster, and blow through it. it should easily blow through the booster side [engine creating vacuum at the proper time]
should seal up [not blow through from engine side], storing vacuum in the booster can.
look at the grommet close on the can to see if there are any cracks and/or if it is supple enough to be able to seal on both the can and the valve.
lastly, easy check for the booster diaphragm, pump it several times with the engine off to expel any residual vacuum, then start the car with your foot on the brake pedal, if the pedal goes down, and the effort seems "easier", the booster "should" be ok.
next would be to rig up a vacuum tester adapter to fit the booster, then use a hand held vacuum pump to introduce 15-18 " into the can and see if it holds.
please let us know how yoy make out.

Booster shot.

thanks for the update ! not too tough of a repair to undertake. hardest part is nut removal from the underdash area.