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I working on reconditioning my brakes and I found a company who sells a seal kit for the trunk mounted pressure differential valve.
This is the brass unit with a switch in the middle that is supposed to detect a brake fluid leak in the rear or front system.
For $8 no excuse to not update it.
It sometimes seizes in normal position so it does not do its job at detecting fluid leaks.
If the seals fail you might find brake fluid leaking from the switch in the middle.
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The "brake pressure differential valve" (aka shuttle valve) is housed in a brass "block". The shuttle valve reacts to pressure differences between the front braking system and the rear braking system. If the pressure differential is great enough, the shuttle valve moves (hopefully) enough to activate the warning light switch (i.e. completes the warning light's electrical circuit to ground).

The brake "proportioning valve" is housed in a cast iron "cylinder".

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