Your master cylinder is already showing symptoms of failure. If you're going to get the RJM pedal, I highly recommend the HD CMC. Installing the new pedal without doing anything about your current master cylinder is likely not going to give you the pedal feel you're looking for. But replacing it with another OEM master that is prone to failure isn't something I would recommend.
The fact that the soft pedal happens more when it is hot makes sense because the fluid thins as it heats up. The thinner fluid gets by the master cylinder internal seals easier.
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2011 370Z 6MT Sport Gun Metallic | ARC | CJM | Ecutek | FI | Fujimura | R2C | SPL | Stillen | TWM | Z1 | ZSpeed | |