W123 Brake issue - PeachParts Mercedes-Benz Forum

Diescipel · #1 09-03-2006, 12:54 PM

I did a search but nothing was specific enough.

78 240D

I have never had to diagnose brake problems--mostly just replace pads, bleed and flush etc. so I need some help so I don't run down the wrong path and spend $$$ unnecessarily.

The passenger front caliper on the 240D seized so I replaced it with a rebuilt caliper along with new pads in both calipers, rotors turned, flushed and bled system yada yada. Immediately I noticed the pedal feel was not as crisp as before. After driving it for a while (ie when the car warms up a little), the pedal will now go to the floor with little brake power.

I figured it was the booster loosing vacuum so I checked that. As far as I can tell there's no problem with the vacuum up to the booster, so I figured the booster was shot. Sound correct? is there a rebuilt kit or are there rebuilt boosters out there?

I seem to remember, though, that if your booster goes (ie vac leak) you should retain the same pedal travel but it will be much harder to push.

your thoughts appreciated.

B

dieseldiehard · #2 09-03-2006, 01:10 PM

I believe you have some air in the lines. What method did you use for bleeding the system? Start with the wheel farthest from the master cylinder, use a power bleed and it should feel the same after bleeding

Motorhead · #3 09-03-2006, 01:11 PM

More than likely you need a master cylinder. When a brake system is working normally, the master cylinder pistons and cups only travel a short distance in the bore. When you bleed the system, you work the master cylinder the full length of its bore. Any rust or build up in the master cyl bore now gets a shot at tearing up the piston rubber cups and causing the master cyl to fail.

jwhughes3 · #4 09-03-2006, 01:26 PM

I agree is sounds like it hasn't been bled completely - but I do not agree with replacing one caliper at a time - I would replace the Master Cylinder and other caliper too and re-bleed with fresh fluid.

John

#5 09-03-2006, 01:38 PM

It does not sound like a bad booster. I would bleed the system again too make sure you have all the air out of the system. It's possible you have a problem with the master cylinder, but that would be quite a coincidence.

BTW, you really shouldn't turn W123 rotors, you should replace them in pairs.

Diescipel · #6 09-03-2006, 05:50 PM

I've bled the system twice to try to eliminate air as the problem. Yes, started with furthest caliper. No, don't have a power bleeder so I did the old pump and hold.

On the second bleed absolutely nothing but clean brake fluid came through.

Replacing both calipers seems excessive, as does replacing the master before I have a solid diagnosis. I'd rather not spend all the $$$ for a process of elimination as a substitute for a focused diagnosis.

If there were still air in the lines I believe it would feel spongy. Instead, as I said, once the car warms up/I drive it a ways the pedal goes through almost the whole length of its travel before I get any actual braking, and then it is difficult to brake. It reminds me a bit of what happens when I don't adjust rear drums on my landcruiser properly.

I'm wondering also if I got a bum caliper or something else happened when the old one seized.

Thanks,
B

#7 09-03-2006, 06:22 PM

Dumb question, are you sure you refilled the master cylinder tank enough to get fluid in both chambers? It needs to have fluid in both halves of the tank.

Diescipel · #8 09-03-2006, 07:27 PM

Quote:

Originally Posted by Craig

Dumb question, are you sure you refilled the master cylinder tank enough to get fluid in both chambers? It needs to have fluid in both halves of the tank.

Good question. I made sure I kept the fluid level up during the bleed, yes, but both chambers??????? I have to look at it more carefully.

This MBZ is a relatively new beast for me to wrench on so I'm still getting used too it.

Thanks,
B

t walgamuth · #9 09-03-2006, 08:05 PM

if you find the mc resorvour full both chambers, then you have a half failed master cylinder. you are pushing through the first chamber of it and engaging only the second chamber.

so if you find the res full front and back, you need a new mc assuming you bled them correctly.

good luck

tom w

estod · #10 09-03-2006, 10:18 PM

There is another possibility that rears its head with front brakes --- many calipers can be switched left-for-right, and will bolt up just fine --- but the bleed screw isn't at the very top of the caliper (if installed on the wrong side). So a pocket of air stays at the top of the caliper and ruins your attempts to bleed the calipers and lines. I may be incorrect that left and right calipers can be installed on the wrong sides on a 123 --- but I did it on Crown Vic wagon and scratched my head for some time before realizing why the brakes wouldn't bleed successfully.

It's easy to check ...

t walgamuth · #11 09-03-2006, 11:12 PM

dunno if you could. the hose would be a problem too i think.

tom w

Diescipel · #12 09-04-2006, 04:17 PM

OK stumped.

Just went and check the reservoir--both chambers are full.

Caliper is mounted with bleed screw up.

Again, I drove the car and the brakes were fine. I didn't drive it very far etc., but I did drive it and I had relatively firm pedal and could lock them up on our fog drizzle covered roads. Modulation was fine.

I'm going to drive a bit farther and see what happens...

B

kmaysob · #13 09-04-2006, 04:29 PM

i would try one more good bleed.ive never really had a problem with bleeding them. when i worked in a shop and decided to flush my brake system on my cars i would put a power bleeder that used suction on and drain the system, then i would have one of the other guys go up in the air while in the car and i would bleed the system from dry with no problems.

Diescipel · #14 09-04-2006, 04:45 PM

Drove about 12 miles, including freeway. No problems. I will drive it 30+ miles to work and back tomorrow and see...

Thanks for everybody's input.

Cheers,
B

t walgamuth · #15 09-04-2006, 06:21 PM

if your mc is failing you may be able to push really hard and go through the first part of it and engage the second part. if this happens you will hit very hard pedal near the floor. at this point you will be engaging the rear brakes only. not good for stopping. the front provide from 2/3 to 3/4 of the stopping power.

tom w