The ability to lock and unlock only some number of times is only a factor on cars using the engine for vacuum and the engine is not running. The 126 chassis uses a vacuum pump and should lock and unlock till the battery runs dead.

The pump is designed to shut off after a certain pressure or vacuum is achieved each cycle. If the system has a leak and the pressures aren't achievable then the pump shuts off after some time period. I suspect something along this line to be your problem. To test locate the pump in the spare tire and watch what happens. Does the pump shut-off after similar intervals both locking and unlocking? If the time is long try removing the line and plugging it and see the time length.

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