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Brake Fluid operation question

Hi guys.

Is brake fluid under any pressure when a car is idling without having your foot on the brake? Or only when brake pressure is appliled?

Thank you so much for your time.

Next question.

Seriously, it isn’t. There’s no pump involved like there is in some other systems.

Should only be under pressure when you apply the brake pedal.

Before ABS, the answer would be no, not under pressure. I think that’s still true, but there may be portions of the system under pressure in the ABS system.

Why do you ask?

I don think so. ABS isn’t activated unless you press the brake pedal.

Thank you for the replies so far guys.

Texases -

My car has a brake leak (pedal goes soft, has happened with this car before).

I can’t take it in to get fixed yet so it will be sitting in the driveway for a few weeks. It will eventually be towed (safety first!).

I think I should be starting the car up every few days and letting it warm up so everything else doesn’t get too “rusty” …

I was just wondering if the brake fluid would leak if I let it idle without stepping on the brakes.

if it’s not leaking with engine off then it shouldn’t leak with engine on.

Personally I’d just let it sit.

Pressure is provided by your foot. ABS only interferes with the hydraulic pressure using a square wave applied to a solenoid-operated valve on the wheel that’s slipping.

Stability control is a different issue. Stability control systems do have the ability to apply the individual brakes. I’m admittedly not as well versed in these systems as I’d like to be, and cannot therefore describe how the force is achieved to do so, so I’ll leave the details to others.

Of course, it depends on how big the leak is. A big leak down by a wheel can gravity-drain some brake fluid.

Just curious, would a power brake booster apply any pressure to the brake lines with the engine running? That’s about the only thing I can think of that would be different engine on vs engine off for the brakes.

Excellent question. Not if it’s working properly.
With no pressure on the pedal, a booster applies the engine vacuum to both sides of the diaphragm equally. When you hit the pedal and the brake rod is moved forward, a valve closes the front of the diaphragm off to only the engine vacuum and vents the rear of the diaphragm, the pressure differential helping you to push the rod that goes to the master cylinder. When the pedal is released, springs in the system pull the rod back and the valve again seals off the entire canister and opens the path between the front of the diaphragm and the rear of the diaphragm, equalizing the pressure.

We’ve occasionally had posters here ask why their brakes continue to apply themselves lightly after the pedal is release, slowly building pressure until it opposes the engine’s efforts. One possibility is that the booster isn’t properly coming back to center. That could cause mild brake drag that could grow as the components heat up. Possible causes are a bad booster or a misadjusted brake rod that doesn’t allow a proper return to null.