Problem with brake system of dodge t110

[serge]

AMERICAN DODGE T110 or CANADIAN DODGE T110 have a brake system that includes a booster and a valve.
The booster has the shape of a cylinder ; at the rear it is connected at all time to the vacuum of the manifold and in front it is connected to the output of a valve.. Inside, there is a large piston rounded by a rubber and a large spring which pushes the piston forward.

When the motor is running, the vacuum in the manifold sets at the rear of the booster and at the entrance of a valve. The outlet of the valve leads to the same vacuum condition at the front of the booster eading the the piston having the same vacuum pressure on each side as long as the brake pedal is not depressed.

When the brake pedal is depressed the output of the valve leads to applying the atmospheric pressure in front of the booster so the piston is moving to the rear of the booster upgrading the efficiency of braking.

As soon as the brake pedal is released, the output of the valve is switched and leads to applying vacuum pressure in front of the piston which moves towards its first position

The problem encountered is the following : the piston does not return at its normal position when the brake pedal is released.
Using a specific tool I have checked the pressure variation at the output of the valve.
( vacuum at the idle state, atmospheric pressure when brake pedal is depressed , vacuum again when brake pedal is released ).

Once having oiled the rubber ring around the piston the system has been working for 40 miles but became out of order again

IPlease is anyone able to help me to resolve the problem?
Regrads Serge