Brake issues with my 300SD - PeachParts Mercedes-Benz Forum

destroy · #1 03-20-2010, 09:22 PM

So, I'm running into some brake issues with my 300SD. I've never had to do much beyond changing pads/rotors/shoes on brakes before, so the rest is still a bit new to me.

I put new pads on the front and in the process of getting the bolts unstuck (PB Blaster, propane torch, cranking, etc...

) we fried the seals in one of the calipers. Enough running around, no good used parts, wrong parts from store, etc... I get the right caliper in and put it on this afternoon.

By now I've bled out more or less all of the old fluid and run over two 350 ml bottles of fluid through the system. After some bleeding today the brakes were getting close to back to normal finally, but I only get a couple of seconds of real decent braking before the pedal fades and softens up a bit. It gets really hard after some pumping but the braking is just not where it should be.

I'm planning on bleeding it more tomorrow, but if that doesn't work anyone got any suggestions?

Tired of throwing time and money at this!

Thanks.

leathermang · #2 03-20-2010, 09:24 PM

How exactly are you bleeding it ?

Skippy · #3 03-20-2010, 09:26 PM

I know in some applications it's possible to put a caliper on upside down, so that the bleed screw is at the bottom rather than the top. Any chance of this?

destroy · #4 03-20-2010, 09:34 PM

Caliper is definitely on the right way. Bleeder screw is on the top.

I'm bleeding it using a helper on the pedal (engine on for power assist) and me on the bottom on the bleeder screw. Pedal down, open the screw - bleed, screw close, pedal release. Repeat.

I'm thinking there's probably air in the rear lines now that I haven't got yet, but we'll see tomorrow.

Skippy · #5 03-20-2010, 09:41 PM

That way should work as long as you keep the reservoir topped up enough that it doesn't suck air into the MC during bleeding.

layback40 · #6 03-20-2010, 09:50 PM

you dont need the engine going.

Skippy · #7 03-20-2010, 09:54 PM

Quote:

Originally Posted by layback40

you dont need the engine going.

Did I miss something?

Edit: Yes I did. I just re-read post #4.

leathermang · #8 03-20-2010, 10:24 PM

Are you doing them in the order of longest measured by the brake line from the mc first?

Diesel911 · #9 03-21-2010, 12:20 AM

Quote:

Originally Posted by Skippy

Did I miss something?

I read on this Forum that when you bleed the Brakes by having someone push on the Brake Pedal without runing the Engine you push further in on the Diaphragm in the Brake Booster. It was claimed that this is hard on a 20 something year old Diaphragm.

I guess running the Engine allows the Diaphragm to operate within the normal range of movement when someone steps on the Pedal and it is less prone to being stressed.

Don't know if this is true or not.

Diesel911 · #10 03-21-2010, 12:25 AM

If Bleeding does not help it may be the Master Cylinder needs replacing. You might have washed out all of the gunk on the bottom that was helping to keep it sealed.

I did not agree with it but I believe about 3 weeks to 1 month ago there was a thread that also said or ask if a Master Cylinder could be damaged by the Manual Bleeding.

leathermang · #11 03-21-2010, 07:43 AM

Quote:

Originally Posted by Diesel911

I did not agree with it but I believe about 3 weeks to 1 month ago there was a thread that also said or ask if a Master Cylinder could be damaged by the Manual Bleeding.

You are kinda taking that out of context .... the idea, which is correct, is that if corrosion has occurred on the shaft where the seal does not usually run....and through BAD CONTROL OF THE BLEEDER NIPPLE .... THE movement of that seal is allowed to run into that area.... you can mess it up....
So it is not the act of manual bleeding...which is still perfectly good....but not knowing and doing it correctly can in fact mess up the old master cylinder.

Diesel911 · #12 03-21-2010, 01:34 PM

Quote:

Originally Posted by leathermang

You are kinda taking that out of context .... the idea, which is correct, is that if corrosion has occurred on the shaft where the seal does not usually run....and through BAD CONTROL OF THE BLEEDER NIPPLE .... THE movement of that seal is allowed to run into that area.... you can mess it up....
So it is not the act of manual bleeding...which is still perfectly good....but not knowing and doing it correctly can in fact mess up the old master cylinder.

I did not remember the exact details of the thread. But, hoped if he was interested the OP would look up the thread.

leathermang · #13 03-21-2010, 01:45 PM

Quote:

Originally Posted by Diesel911

I did not remember the exact details of the thread. But, hoped if he was interested the OP would look up the thread.

I just did not want people to be left with the impression there was something inherently wrong with the standard brake bleeding process done correctly....
and it has some advantages over power bleeding... the amount of pressure is typically much greater since the PSI capacity of the fluid holder part of the master cylinder has to be the limiting factor....as compared to the working parts which deal with high pressures regularly and may need that to flush junk out. Done correctly it also typically uses less brake fluid.

Cr from Texas · #14 03-21-2010, 02:16 PM

I've always used this bleeding method (for about 50 years) with no problem but I've never done it with the engine running. Try it with the engine off, and like leathermang stated, start witht he longest line. I've always let the pedal go to the floor and have never damaged a MC. But to be safe, you can put a block under the brake pedal to limit travel.

leathermang · #15 03-21-2010, 02:21 PM

Since I am always the one on the ground... I do not have the engine running either....
The distance the pedal goes is entirely in the control of the person opening the bleeder nipple... it is only open a fraction of a second.. a fraction of a turn... you want to close it while the pressure is still high in the fluid....

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