Brake bleeding issue on a W113 Pagoda - PeachParts Mercedes-Benz Forum

GGR · #1 01-15-2011, 02:01 PM

Hi all,

I started the year in fanfare by buying a '71 280SL at an estate auction. The car needs some work but has very little rust which is what I was interested in. I drove the car home 100 miles from the aution. It worked well but the car slightly pulled to the right when braking hard. So I thought I would replace the brake fluid and bleed the brakes before taking it to inspection.

The car came without history but the brake fluid reservoir, the master cylinder and brake hoses look new. On the other hand the brake fluid was very dark. I found some ten years old service bills with the mileage on it (not brake related) showing that the car did less than 1000 miles in ten years. It looks like the car had some work done on it ten years ago, and this is when these brake parts may have been replaced. The car then sat in a garage without moving for the following ten years explaning how the brake fluid got dark.

So this morning I sucked all the old brake fluid out of the reservoir and then cleaned the interior with a cloth. I poored new brake fluid and went to bleed the rear right calliper. At the beginning some air came out with pressure when my friend was pressing on the pedal but then only very little liquid came out with no pressure. I put the wheel back on and the pedal is very loose. I still have a bit of brakes toi the front but I wonder why I was unable to bleed the rear? Do I have a blockage somewhere? If this were the case pressure would build up and the pedal would be hard, which is not the case. It's as if the master cylinder had gone wrong for the rear brake circuit. Any idea?

Thanks in advance!

GGR · #2 01-15-2011, 02:31 PM

There is a bolt in the middle of the master cylinder at the bottom. Is that a bleeder? Sould I bleed that first? I don't understand how air could have gotten in the master cylinder as I did not remove the reservoir and I refilled it with new fluid before pressing on the pedal.

I also noted that the liquid in the front part of the reservoir has gotten darker as it must have mixed with some older fluid below, while the liquid in the rear part of the reservoir has remained clear showing that it is not communicating with the rest of the rear circuit.

Carpenterman · #3 01-15-2011, 03:57 PM

It is possible that you got air into the rear line while bleeding it. Did you close the bleeder at the end of each pedal stroke to keep from sucking in air? Does the pedal pump up at all?

Or, it's possible the inside of the rear brake line(s) have collapsed, explaining why you could no longer get fluid out of the bleeder fitting. To complicate things, if the master cylinder reservoir is completely full and the pedal won't pump up to a hard state, the master cylinder may have failed while you were doing the bleed because of the longer travel of the piston into the unworn portion of the bore.

Both the collapsed rear hose and the failed master cylinder while bleeding have both happened to me (why I finally made a power bleeder). Try bleeding all four again insuring that the reservoir remains completely full. If that doesn't work, I would replace the rear hoses. If you still have a soft pedal, the master cylinder may be shot.

Benz Dr. · #4 01-15-2011, 05:45 PM

Once in a while it ends up being the brace force regulator but that's pretty rare.

Sounds like a MC problem. There's a seal on the cylinder that keeps air from entering and once that's damaged every time you hit the pedal it will draw air into the brake fluid. You can bleed forever and never get all the air out.

Get a new MC and don't try and rebuild it.

SilkMoneyLove · #5 01-15-2011, 06:27 PM

Sounds MC related or else you would be getting pressure. Any leaking anywhere? Since it is already boogered, I would keep pumping the brake to see if I could get pressure to build up. Maybe have the car running to use the brake booster to help with pressure?
Maybe wiping the MC Res left some lint and clogged something?

GGR · #6 01-15-2011, 10:22 PM

Thanks all.

If it were a collapsed hose I may have little or nothing coming out of the bleeder but I would be able to build pressure which is not the case. Thinking about it the rear piston (for the rear circuit) in the MC may not come back fully in place. It may be pushed back by the first piston to a certain point and then stay there. That would explain why the fluid in the rear of the reservoir does not communicate with the rest of the rear circuit. I may take the MC out and see if there is something wrong with it. Why shouldn't I try to rebuild it as long as the bore is not damaged?

WANT '71 280SEL · #7 01-15-2011, 11:29 PM

For starters because they're usually so cheap. Plus, if you try to rebuilt it, you'll have a hard time removing any rust or scale without messing up the sealing surface, making your rebuild job fruitless. Go to your local McAuto Parts store and inquire. I think you'll be pleasantly surprised.

Best Regards,

David

alabbasi · #8 01-16-2011, 01:12 PM

Had a very similar issue with my 6.3 just this weekend. Turned out that after replacing the master cylinder, I was able to bleed the back brakes.

GGR · #9 01-16-2011, 07:04 PM

I took the master cylinder apart. I had a bit of difficulties to get the pistons out but after a bit of cleaning all looked like new, which it was ten years ago. That thing was replaced and then to car got into storage. I put it back, replaced pads on three callipers and pistons went back nicely. The night cought me before I could do the fourth calliper. Will finish tomorrow and see if I can now bleed the rear brakes.