The first thing to check is the rubber bladder seal under the reservoir cap. If it's blown up and mushy, there' a petroleum-based contaminant in the brake fluid. The only proper repair for that is to replace every part in the hydraulic system that contains rubber parts that contacts the brake fluid, and all steel lines must be flushed and dried.
At 52,000 miles and six years old, it's very unlikely the master cylinder failed on its own. If it failed due to fluid contamination, that contamination will leach back out of other seals and hoses and recontaminate the new master cylinder. That typically takes a few weeks to a couple of months.
The way to prove or disprove this is to loosen the steel lines right at the master cylinder while the brakes are locked up. If they release, fluid contamination is the chief suspect. The misadjusted push rod or brake light switch that I mentioned previously wouldn't be okay for three months, then suddenly start causing a problem.
Monday, September 12th, 2011 AT 5:28 AM