Reviving the thread - looks like there are many of these observations about that relief valve for the Polaris cleaners - I have a 380, and after some years, the water coming upwards from that relief valve has increased, with that came a slow down of the cleaner.
The booster pump is the same and I have taken apart, from the hook up at the outlet in the pool wall, to the cleaner itself with replacement of the bands that move the wheels.
In one of the many threads about this subject, someone, after replacing the relief valve twice, took it out and put a plug - Polaris will of course say extreme and may damage the internals of the cleaner, but the poster claims that it brought the cleaner to its original speed and performance.