Look on the cap. MB caps have come with 100, 120, and 140 on them. This refers to 1bar, 1.2 bar and 1.4 bar. A bar is 14.2 psi. The system must be able to withstand the pressure that the cap releases at. If the cap does its job thats the highest pressure it will ever see.
Bosch Master, ASE Master, L1
33 years MB technician