The invention concerns a method for the optical analysis of a fluid, in which the fluid that has a first refractive index, is surrounded by an auxiliary fluid with a second refractive index, the first refractive index being larger than the second refractive index and light being sent through the fluid. The brake negative pressure may be supplemented without increasing the capacity of the vacuum pump to improve the braking performance. A current latency level can be dynamically calculated using the performance statistics and compared to previously established latency threshold levels.