Weird brake pedal dipping when coasting to a stop

[primo107]

Lets say I am doing 80kmh and the light turns red but its still far away - I just touch the brake and hold it slowing down little by little and at some point the pedal looses a bit of pressure and dips like 10-20mm - the car is an automatic 323ti and it is also downshifting during this process. Could this be related to the way the transmission works or does it get a brake booster leak or something?

[bluptgm3]

I’m guessing the throttle off downshift increase in RPM raises vacuum pressure in the booster causing the pedal to sink.
Now somebody weigh in and dispel this theory.


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[JDStrickland]

My money is on a failing Brake Master Cylinder.

TEST
Hold your foot on the brake pedal while the car is stopped and see if the pedal falls. You don't have to mash the pedal, just hold it normally as you might while waiting at a red light, perhaps with a slight downhill. The car tends to not roll on flat ground, so you may not use enough pressure, but if the road was a slight downhill then you will hold the pedal a little more firmly. If the pedal sinks, then you are experiencing an internal leak of the master cylinder, the fluid is leaking past an o-ring and allowing pedal movement. There is no leak to the outside -- you will not see a puddle of brake fluid anywhere -- it is entirely an internal leak.

The corrective action is to replace the brake master cylinder. You do this from inside of the engine bay.

You MUST perform a procedure called Bench Bleeding of the brake master cylinder before it is will be serviceable. They sell a kit of hoses that you connect to the ports where the brake pipes connect. Place the open end of the hoses into the reservoir cavities, fill the cavities with brake fluid, then compress the piston that connects to the brake pedal several times until the bubbles stop. You must secure the master cylinder in a vise, then use a screwdriver or other suitable tool to fully depress the piston. The deal is, the piston can move more than the pedal will push it, if you do not bench bleed, then there will be an air pocket within the master cylinder that will never go away. This will give you a squishy pedal and poor braking performance. After you mount the master cylinder, you can remove the hoses one at a time and connect the respective brake pipes as you go.

[JDStrickland]

There is a master cylinder and four wheel cylinders -- brake calipers. If the calipers leak, there will be fluid that leaks to the outside. You will observe brake fluid on the insides of the tires where the calipers leak. this could be one or more corners of the car. When the master cylinder leaks, it will not leak to the outside. That is, there will be no visible brake fluid. The master cylinder has multiple chambers inside and o-rings that seal the passages as the piston moves. Your issue is that the o-rings have worn out and fluid is allowed to leak around them and go from one chamber to another. The pedal falls as this happens. You can recover from the leak by lifting your foot and pumping the pedal. Eventually, you cannot recover and brake failure can occur.

When you finish replacing the master cylinder, you will have to bleed the entire brake system. A pressure bleeder is the best way to do this.

[primo107]

Tested holding down the pedal in various situations - no pressure is lost only while the car is downshifting during slow stops. Could the servo not have enough power during the operation of the auto transmission?