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Diagnosing residual brake fluid pressure:
 

Symptoms of residual brake fluid pressure are dragging brakes that become hot. Sometimes it is significant enough that a driver immediately feels the dragging. Sometimes it is minor enough you don't feel it.

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The most common causes of residual brake fluid pressure are:

  • preload on a booster's input rod

  • an out of adjustment booster output rod (not applicable to all boosters)

  • a misaligned pedal to booster rod connection

  • A faulty master cylinder rarely causes this condition but it does happen

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Here is how to diagnose:
-put the axle(s) experiencing the symptoms up on jack stands and see if you can reproduce the problem up on the jack stands. Sometimes these problems only occur when the truck is warm, and if that is the case, warm the truck up before putting it on jack stands. Turning the tires by hand for the next part probably works best. Try to rotate the tire and verify the dragging of the brakes. Get a feel for the characteristic of the particular dragging / residual pressure issue before moving on.

 

-Loosen the two nuts holding the master cylinder to the booster so that the MC can come outward at least 3/8" and feels loose. Check the tire rotation again. If this causes the dragging to go away, that means the master cylinder is holding the residual pressure but it is not necessarily a fault of the master cylinder.
 

-If the above MC loosening does not cause the residual pressure to go away, loosen the tube nuts holding the brake lines to the master cylinder. They don't need entirely taken off, just enough for fluid to leak out. If this causes the dragging to go away, that means the master cylinder is holding the residual pressure and it is the master cylinder at issue and needing replaced.
 

-If the above MC does not cause the residual pressure to go away, check for preload form MC to booster by removing the two nuts holding the MC in place and gently holding the MC to the booster to verify the booster's output rod is not pushing on the MC' s plunger. You would be able to feel this by hand. The MC should go up flush to the booster with no spring-like push back. If there is push back, this is MC preload. Sometimes this condition only occurs when the engine is running so if you first get no preload, test again with the engine running.

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-If there is MC preload, press the brake pedal three times with the engine off. This will cause the stored vacuum or steering fluid pressure to be used up and no longer be present.

-If pressing the pedal 3 times with the engine off caused the issue to go away, this means the booster may be defective. Still review the possible issues in the next paragraph because it's an overactive/ defective will give nearly identical symptoms as a perfectly fine booster with a mix of the issues in the following paragraph. If the issues in the next paragraph can be ruled out, it is likely a defective booster.

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-If there is MC preload, the next things to look for are:

  • preload on the input rod of the booster where it connects to the pedal. Some considerations are:

    • brake light switch adjustment

    • incorrect booster mounting brackets that are too shallow and hold the too rearward booster

    • incorrect booster type with too long of a rod

    • Booster input rod out of adjustment (where applicable)

  • friction from misaligned booster rod to pedal connection. This can cause the pedal to not properly return. A visual inspection with a flashlight can help verify this connection is square and not binding. Sometimes a booster's input rod wants to be on on side of the pedal and if it's on the other side, you get friction.

  • The output rod of the booster being too long from incorrect adjustment or improper manufacturing. Some booster's have an adjustable output rod and if so, the rod can be turned clockwise until there is zero preload

 

-If there is not MC preload, and the residual pressure does not go away with loosening the MC or the brake lines at the MC, start looking downstream of the MC. This could possibly be a kinked brake line or a faulty proportioning valve. It is fairly rare for a proportioning valve to fail in this way.

If this does not diagnose the issue, please reach out with a summary of what you find to help us diagnose

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BBB sells components to upgrade braking and suspension systems on classic vehicles

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