126 Sinking brake pedal
 

I have gone through the search section to resolve the sinking pedal issue . Like to hear from those who successfully rectified this fault .
Thank you
mak
300se 89

1. Master Cylinder

2. Booster

3. Worm brake lines(rubber)


Booster check - depress brake pedal - hold it there - no need for extreme pressure - while holding brake pedal down - start car - brake pedal should fall thru a bit - if so - booster likely OK.

Look at brake hoses - any gettin' fat? - no? - next line


Master Cylinder

Just went thru this on my 86 300sdl. It was the master cylinder. Even though I had no fluid leaking anywhere, it was the master cylinder. I stubbornly replaced all my rubber brake hoses first as they were 24 years old and needed replacing, but my pedal still would sink slowly while sitting at a red light. I replaced the master cylinder and all is well.

Booster problem will exhibit themselves as very firm pedal, hard to press down.

Master problems will show by the sinking pedal.

Hose problems will show by squishy pedal.

I THINK. I too am working through some brake pedal issues. First up is replacing the soft lines. Mine are stainless steel lines that have been on there since I bought the car so 5 years + however many before that. Potentially another 5-7...

Well, if there are any leaks that would be the cause. And, you will find your fluid level slowly going down. If there is not a leak in the lines somewhere, then your master cylinder is most likely bad. I had this happen in one of my 300SDs, fluid was leaking into the brake booster. IF there is fluid at the bottom of the brake booster, replace both the MC and the booster, as brake fluid is extremely corrosive and will eventually cause the BB to rust. Used BB is the way to go in that case, as new they are very expensive! Replacing the MC - hour or 2, BB - an hour at most. Bleeding and making sure there are no leaks - a few hours, be meticulous!

new MC
 

When i had the sinking pedal problem on the 300se ,the MC was duly replaced . its still the same ,slow sinkage at the lights . i suspect the ABS unit may be harbouring a pocket of air bubble, though i had pressure bled the system .
is there a bleed procedure for the pump
Thanks
mak

The ABS pumps have accumulators, spring loaded cylinders holding brake fluid, inside. Depress the brake pedal 60 or 70 times to relieve the pressure. Then bleed the brakes.

I also recall a page in the service manual detailing a brake bleed screw on the pump body. I forget the procedure.

Bosch ABS2, which is what our 80s non ASR 'benses have, do not have pressure accumulators. Just a return pump and valves. ASR equipped W126s do.

On non ASR cars bleed the brakes as you would normally. There is no extra bleeding needed to clear air from the ABS system.

The problem here is probably the master cylinder, but it might be possible for the ABS system to cause a sinking pedal if it were broken somehow. Unlikely though.

-J

If you catch it soon enough, you can flush out the brake fluid accumulated in the booster and "save" the booster.

Jim