1986 300SDL Brakes Driving Me Insane

[gehunter]

I just changed the front and rear brakes on my 300SDL. PITA job. Entirely swapped out with new rotors, ceramic pads, and calipers.

After getting everything connected up, I was bleeding the front brakes (old school method -- pump and hold). It worked, and I got all of the air out from the front.

The rear brake however...nothing. Can't get any air out of that thing and the pedal never seems to sink fully to the floor.

Called it a day. Went outside later, just to test the pedal. And it sank right to the floor. Fired it up took it for a driveway test drive, and it's like there's no brakes at all.

The brake fluid light is on. The reservoir is empty (bone dry kinda empty) and it wasn't that way this afternoon. No obvious visible sign of a leaks A little dampness on the garage floor, but it doesn't look like an entire reservoir of brake fluid. There's gotta be a leak somewhere.

Any thoughts???

[Mark DiSilvestro]

If there's absolutely no sign of an external brake-fluid leak, but you lost that much fluid, it's possible you blew the master cylinder seal that faces into the power-brake booster.
An old master cylinder may develop internal rust or dirt build-up past the limit of it's normal piston travel.
Sometimes, pushing the pedal past it's normal travel, can damage the old piston-seals, by forcing them into this rust or dirt.
Unless it's unavoidable, whenever I do any brake work, I try not to push the pedal all the way to the floor. If I've opened the hydraulics, I'll gravity-bleed the system for awhile. Then I can usually use short pedal-strokes to pump up the brakes or seat the pads.

Happy Motoring, Mark

[dude99]

If there's absolutely no sign of an external brake-fluid leak, but you lost that much fluid, it's possible you blew the master cylinder seal that faces into the power-brake booster.
Because an old master cylinder may develop rust or dirt build-up past the limit of it's normal travel.
Sometimes pushing the pedal past it's normal travel can force the seals into this rust or dirt, damaging them.
Sometimes it's unavoidable, but whenever I do any brake work, I try not to push the pedal all the way to the floor. If I've had the hdraulics opened, I'll gravity-bleed the system for awhile. Then I can usually use short strokes to pump up the brakes or seat the pads.

x2

[TX76513]

Just a thought on the W126 you have to overfill the(main) reservoir so it flows to the rear. The main compartment is front brakes and its very difficult to see the rear compartment.

Remember you have ABS -- power the car or power the car through each bleed point.

[DieselPaul]

If the car has ABS you often need to use a power bleeder, the FSM says to bleed a 124 you need something like 2bar of pressure.

I've had this problem on my ABS equipped Mercedes and I've always just used a power bleeder and that solves the problem.

[barry12345]

You did put the replacement calipers on the right sides at the rear. The bleeder has to be at the highest spot on the chamber or you will never get the air out. Just mentioned this in case but assume you have them on their appropiate sides.

[pawoSD]

Sounds like you need a new brake booster and master cylinder....

[Brian Carlton]

Originally Posted by TX76513

Just a thought on the W126 you have to overfill the(main) reservoir so it flows to the rear. The main compartment is front brakes and its very difficult to see the rear compartment.

This is the problem.

The rear reservoir is empty. If you look at the reservoir from the driver's side, you believe that it is full.

If you look at the reservoir from the passenger side, you will be shocked to find that it is EMPTY.

[Brian Carlton]

Originally Posted by pawoSD

Sounds like you need a new brake booster and master cylinder....

This is a factually incorrect statement made by a person who should know better.

[gehunter]

Thanks to everyone for their suggestions. I've put together a top 10 checklist of points I'm going to hit, listed in terms of decreasing desirability, to see if I can solve this myself. I'll keep you posted. Here's the list:

1. Power the car (I do have ABS). In theory it is working, no reason not to
2. Overfill main reservoir so that it flows to the rear, I had no clue about that.
3. Incline car facing slightly uphill/ Gravity bleed the system/ light pumping action (again, no clue about that trick)
4. If that doesn’t work, get a better power bleeder (mine is rather wimpy, might have to turn to the “pros” for that)
5. I’ll double check the calipers, just in case I’ve lost my mind and done something completely bass-ackwards
6. Get a new master cylinder, install it
7. Get a new brake booster, install it
8. Hire a voodoo priestess
9. Sacrifice a chicken/ sell soul to satan (see law of diminishing returns)
10. Buy a Toyota (kidding)

[Stretch]

Originally Posted by gehunter

Thanks to everyone for their suggestions. I've put together a top 10 checklist of points I'm going to hit, listed in terms of decreasing desirability, to see if I can solve this myself. I'll keep you posted. Here's the list:

1. Power the car (I do have ABS). In theory it is working, no reason not to
2. Overfill main reservoir so that it flows to the rear, I had no clue about that.
3. Incline car facing slightly uphill/ Gravity bleed the system/ light pumping action (again, no clue about that trick)
4. If that doesn’t work, get a better power bleeder (mine is rather wimpy, might have to turn to the “pros” for that)
5. I’ll double check the calipers, just in case I’ve lost my mind and done something completely bass-ackwards
6. Get a new master cylinder, install it
7. Get a new brake booster, install it
8. Hire a voodoo priestess
9. Sacrifice a chicken/ sell soul to satan (see law of diminishing returns)
10. Buy a Toyota (kidding)

Well number 10 isn't a bad choice in my opinion, and number 8 could be fun for a different reason...

...the only thing that concerns me is that it sounds like you are about to chuck a lot of money at the car. In my experience that is usually just expensive and it doesn't necessarily solve the problem.

Just try the power bleeder you've got first - then you only waste some brake fluid if it doesn't work.

[TX76513]

Originally Posted by gehunter

Thanks to everyone for their suggestions. I've put together a top 10 checklist of points I'm going to hit, listed in terms of decreasing desirability, to see if I can solve this myself. I'll keep you posted. Here's the list:

1. Power the car (I do have ABS). In theory it is working, no reason not to
2. Overfill main reservoir so that it flows to the rear, I had no clue about that.
3. Incline car facing slightly uphill/ Gravity bleed the system/ light pumping action (again, no clue about that trick)
4. If that doesn’t work, get a better power bleeder (mine is rather wimpy, might have to turn to the “pros” for that)
5. I’ll double check the calipers, just in case I’ve lost my mind and done something completely bass-ackwards
6. Get a new master cylinder, install it
7. Get a new brake booster, install it
8. Hire a voodoo priestess
9. Sacrifice a chicken/ sell soul to satan (see law of diminishing returns)
10. Buy a Toyota (kidding)

You have the general idea - the Voodoo Priestess is not a bad idea though. Never used a power bleeder. With some well placed rags you can avoid any messes. On #5 I have done that - Jeeze. Good Luck

[BobK]

bleeding order:
right rear
left rear
right front
left front
always start with the bleeder furthest from the MC. Keep bleeding until no more air bubles and clear fluid. Best to have clear hose on bleeder going into a clear container with some fluid covering the hose at the bottom of the container. Try not to push pedal all the way to the floor (too late, I guess). I tend to use quite a bit of fluid when doing this just to be sure I have clean, fresh fluid and no air in system.
BTW, I have two '86 SDLs. I'd rather do front rotors all day than doing rear e-brake parts.

[sassparilla_kid]

I'm pretty sure you're supposed to bleed brakes furthest from the reservoir first, closest last, as a general rule of thumb

Derp I just saw that's what the comment above mine said, hahaa

[pawoSD]

If using a power bleeder make super sure the little rubber caps on the level sensors of the reservoir are soft/in good shape, or its going to erupt with fluid when under pressure. If they have not been replaced yet, they are going to rupture/leak when you pressurize it, then brake fluid will be everywhere.