brake fluid dot 5 to dot 4

[ERA174]

The following is in addition to the previous posted comments. My understanding, given I’m not a brake system expert: DOT5 doesn’t absorb moisture, where DOT3/4 will absorb moisture. All hydraulic brake systems ingest moisture from the atmosphere. The moisture ingested into the hydraulic brake system is detrimental to the system. Moisture in small quantities and at less than extreme braking conditions is not a significant degradation to the performance of the system. However, over time as the moisture increases in the system it has an increasing negative impact. For this reason, brake fluid either DOT5 or DOT3/4 need to be flushed periodically with clean fresh fluid of the same kind as previously in the system. DOT5 is not compatible with DOT3/4, which are usually compatible with each other. DOT5 doesn’t absorb the moisture and it will settle to low points in the system, usually the calipers. The pool of moisture in the low point of the system, if not flushed can corrode the system where it pools. DOT3/4 absorbs the moisture, which keeps it from pooling, although still detrimental. If it were me, assuming no existing damage to the system or performance issues, I would sufficiently flush the DOT5 with new clean DOT5 and continue to do so every 3 years. When I have transitioned a brake system in the past from DOT5 to DOT3/4, I always completely rebuilt all the components in the system, eliminated all the previous fluid and then introduced the new non-compatible fluid.