W124 brakes - too much pedal travel - bad M/C?
 

I recently purchased a 1987 300D 'fixer upper' that I knew needed a
brake job due to the flickering dash light, and also what I considered to be
poor feel at the pedal. The pedal would go fairly low before there was
'good' feel, then it was OK - you just had to move the pedal much more than
normal before the car started slowing (compared to my other 1987 300D, and
most other W124's I've driven.) I expected that new rotors/pads & bleeding
would cure it. (Silly me.)

So, I replaced all 4 rotors & pads, and as preventive maintenance
replaced the sorry-looking old rubber hoses with new ones, and pressure bled
the system with fresh fluid. And what did I get? No dash light, but the same
excess pedal travel I had before! What would cause this? Everything else
seems OK, all that's left (that I can think of) is the master cylinder or
ABS unit - and I don't see how either would cause this symptom. Usually
m/c's fail in other ways. (??) I know the rotors are straight, calipers seem
peachy, pads/rotors are properly cleaned and bedded in, and there is no air
in the system. What's left that would cause a low pedal? I don't want to
spend $100+ for a m/c if I'm going to get the same problem afterwards...
Anyone BTDT?



Thanks!

Dave M.
Sacramento, CA
1987 300D - 226kmi (great brakes)
1987 300D - 230kmi (lousy brakes - but WHY?)
1983 300D - 302kmi (pads feel like wood, but work OK)
1983 240D - 203kmi (for sale - great brakes)

brake problems.
 

I don't know if this will be too much help to you. But I used to have the same problem on my 1980 bronco, where the pedal would travel all the way to the floor before my brakes would kick in. Once I had to stop with the e-brake to prevent crashing over the curb and into my neighbors lawn. A new master cylinder fixed it.

First: Has the m/c and/or brake booster been replaced? If so, you may have a mechanical problem rather than a hydraulic one. Perhaps there is excess clearance in the linkage from the booster which activates the m/c?

Second: I had a similar problem with my 1987 300TDT, though it occurred *after* a brake service & bleeding.

I had used a homebuilt pressure bleeder to flush out the old fluid. Said bleeder used pressurized air to move fluid from the reservior, through the m/c, and out to the calipers. Despite the use of several liters of brake fluid, more than a few curse words, and a ritual sacrifice to Stuttgart, I could not get a firm pedal. The brakes didn't even really work.

So I threw the darn thing away and did it the old fashined way. I attached a rubber tube to the bleeder nipple of the chosen caliper and placed said tube in a jar of brake fluid. Opened nipple and proceeded to pump away on the brake pedal. There were two odd results of this technique: 1) It didn't draw in fluid from the jar when releasing the pedal; it all came from the reservoir 2) It worked perfectly.

Turns out the bleeder was forcing tiny air bubbles into the fluid, which were then drawn into the m/c and throughout the system.

If you give this old technique a try, be careful not to empty the reservoir. It only takes about 5 pedal pumps to completely empty the reservoir for the rear brakes. Be sure to wait a few seconds between pumps for fluid to flow from the reservoir to the m/c.

Some poster's will likely express concern about using the pump-the-pedal method because it occasionally ruins the seals in the m/c. While this is a legitimate concern, it seems you don't have much to lose.