If you mean the compensator valve which is the stalk with a spring around it, that is the suspected cause of most BTV complaints of stop at one corner but not the next sudden failures. That little blob of rubber is the only thing holding the fluid in when brakes are applied. If it doesn't seal for whatever reason there is pedal to the floor and no brakes.
The relief port & gasket issue is shown in the following photos borrowed from an early discussion.
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