Your description of symptom could be interpreted as an unintended ABS application. If it happens below 10kph then it won't be recognized as a fault. If you wish to check the system or relate to a real application, find a grassy field and drive at about 5mph and slam on the brakes. The pedal should pulse at you. A single ABS presure intervention can seem like the brakes released.

If this seems to be the issue a speed sensor will likely be the problem and a close look at their signals will be required to evaluate.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician