1994 C280 - Spongy/Soft Brakes, Dashboard Warning Brake Light On, Is it Master Cylinder or Brake Booster problem?

[Arv]

Hello,

Please help figuring out exact problem.

Spongy/Soft Brakes, Dashboard Warning Brake Light just came On yesterday
after replacing the front brake pads.

Brakes were spongy/soft for several weeks but were working fine. I thought since
the front brake pads were little worn out (were about 35% left), I am feeling pedal
little soft.

280, 000 miles on the car.

Just replaced Front Brake Pads.
Front Brake Discs are almost new.
Rear Brake Pads are still about 60% left.
NO OIL LEAKS, oil level in the reservoir it MAX.

Even after pumping the pedal slowly many times, no firm feeling, pedal goes
to the floor. But Still stops the car very slowly.

Disconnected the battery to see if the Dashboard Warning Brake Light goes
away, but didn't. It came back on/stayed on after connecting the battery.
ABS Light doesn't stay on permanently after starting the car.

Is it Master Cylinder or Brake Booster problem or something else? I am little puzzled.

Thank you for all help in advance,
Arv.

 

[Uncle Benz]

I’d try bleeding them first

 

[Arv]

Ok, thank you for your suggestion. I was thinking of air but considering
it happened all of a sudden, was wondering.

Thank you,
Arv.

 

[Uncle Benz]

Very, very occasionally I’ve had air get in when pushing the caliper piston back. Bleeding will resolve it. It maybe you have a master cylinder failure, but I’d bleed it first to see.

 

[Arv]

Ok, thank you again.

What precautions should I take?
How to ensure all air gets out?
How much new oil will I need?

Thank you again,
Arv.

 

[Uncle Benz]

Haha! Have a litre of DoT4 fluid to hand, a thin bore tube, an empty bottle and a friend with a trusted foot
Have a look on YouTube for some guidance if you haven’t done it before

 

[Arv]

Thank you. Yes, I will check some videos out.

Will bleeding the brake system this get rid of the dashboard Brake warning light as well or I will have to go to a mechanic to get it reset?

I tried disconnecting the battery to clear all codes, but it is still on?

 

[Uncle Benz]

What does the light look like?

 

[Arv]

It's not ABS light. The dashboard warning light is "a circle with 1/4th size circular pads (one on each side) around it, like drum brakes."

 

[Uncle Benz]

If red, it’s the brake fluid warning lamp. If amber it’s pad wear indicator

 

[mioba]

could well be indicator not plugged in - did you buy a new one and replace.
But spongy brakes is more attributed to high compression of air in the system.

 

[Arv]

Thank you both.
Amber color.

I just used the existing old brake pad wear sensor. The front pads are brand new. The older ones were still
about 30% left when the brakes became soft.

Bleeding the brake fluid should be done with engine off while slowly pressing
the brake pedal, right ? Just making sure.

 

[Arv]

Problem fixed friends. Thank you for all your help. The brake is firm now.

Mistake was mine to start with. When I had replaced the front pads, I had
forgotten to tighten the reservoir cap all the way & I drove 15 miles
with the cap loose exposing the system to atmospheric pressure.

The passenger side rear calliper was full of air. I was shocked to see how
much air I took out. I used a 60ml syringe to suck air out & I had to
suck out all the way 4 times before oil came out of the calliper.

Then I opened the driver side calliper drain & oil oozed out immediately.

So the Rule of Thumb is,

if oil oozes out immediately when drain is loosened slightly,
that means there is NO AIR but if NO oil oozes out, you got air
in the calliper for sure.

And the brake wear dashboard warning light seems to be because of the sensor
may be grounding somewhere. I will have to check the sensor wire.

Thank you Uncle Ben & Mioba for your help. Appreciate it.
Arv.

 

[Uncle Benz]

Sounds unorthodox, but a win is a win. Well done!

 

[Arv]

Thanks Uncle Ben. But I didn't quite understand what you mean by unorthodox.