Info needed: Bendix brake booster testing

[Alastair Thomas]

Our F60S has been suffering a loss of power so, to cut a long story short, we rebuilt another engine and fitted it. The truck is still gutless so, wiping the egg off our faces, we are now puzzling as to what the cause could be.

Having been through the petrol/carb side and the ignition setting and timing we thought we would see if there was a leak in the inlet side.
This lead to our taking the Bendix brake booster off and examining it.
I removed the can from the front casting and could see nothing wrong with the spring, piston or leather diaphragm.
I do not know how to test these things on the bench.
I reasoned that since the principal of operation is to have equal pressures on either side of the piston when not operating (normally 4 to 5psi but normal air pressure in my test) and then introducing a higher air pressure to the front connection. Operating the brake linkage, the piston should move against the spring. (See picture).
Nothing happened except air coming out of the four holes in the actuator.

Could anyone advise me where I have gone wrong please?

Alastair
F60S LAAT
Ford Lynx I MkIII*

Servo test [2].jpg