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1988 560SL, 2005 BMW X3
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Discussion Starter · #1 ·
Just recently replaced all brake pads, rotors, and rear brake calipers. When I start the car, the brake pedal gets pretty firm when I pump it, but once I put it in Drive, the brake pedal gets soft and almost goes all the way to the floor. I do have some braking power, but very little. I bled the brakes, but they probably could have been bled A LOT better.

I had the brake master cylinder replaced a little under a year ago, but could it have already failed? Or would getting a much better brake bleed solve the problem? I have a hunch that the former would be the problem. Did I just answer my own question? :surrender:
 

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'01-E320 & 02-ST2
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When you pump it and have a firm pedal, if you simply hold pressure on the pedal, does it stay firm? Or does it slowly sink to the floor? If the latter, then it's the master cylinder. If it stays firm once pumped up while you're holding pressure on it, then you have air in the lines.

And how was the pedal before the work? If it was fine before, then that is probably your answer. ;)

Particularly since you don't sound sure of your bleeding efforts, I'd refocus my efforts there. If it doesn't improve then I'd look at the rear calipers to be sure there isn't an issue with them.

Take care and enjoy the ride,
Greg
 

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1988 560SL, 2005 BMW X3
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Discussion Starter · #3 ·
I just went back out and tested it. With the car off, I can pump the brakes and it gets pretty firm, but it does slowly sink to the floor as I apply pressure. When I start the car, it becomes soft and pumping the brakes does not do anything.

Come to think of it, the pedal was very soft and went to the floor before I replaced everything. It began sometime in mid February and has been sitting ever since then. So, the master cylinder is mostly likely the problem?
 

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MC06A said:
...it gets pretty firm, but it does slowly sink to the floor as I apply pressure.

That is a classic description of a failed master cylinder.









Or, erectile dysfunction. Take your pick.
 

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1988 560SL, 2005 BMW X3
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120 Posts
Discussion Starter · #5 ·
That is a classic description of a failed master cylinder.

Or, erectile dysfunction. Take your pick.

Haha, touche, good sir.

I forgot to add/edit something to the last post. With the engine off, I can pump the brakes and hold pressure on the pedal. While it does slowly sink, it only goes about halfway to the floor. If that makes any difference at all.

Should I go get the brake fluid flushed (I think I'm overdue on that anyway) and see if that will fix it, or should I just get the master cylinder replaced at the same time as the fluid flush?
 

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450SLC
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If it is a problem with the master cylinder then don't just buy a new one look around for a place to install a stainless steel sleeve in yours as this will stop you ever having problems with the Master again. Pump bleeding these cars is about the worst thing you can do with the old Master cylinders because it uses the 70% or so travel thats never used and is always rusted or gunked up, best way is to pressure bleed it or gravity feed it.

If you get a shop to do the flush they will be able to tell you before they flush it if the Master is gone, also they should be able to arrange for a group to rebuild your Master cylinder with a stainless sleeve in it.
 

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Always Remembered RIP
1973 450 SLC AMG, 1995 Chevy Tahoe
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Just sound like bleeding them with a friend will do it to me.
 

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1979 280SL, 1984 280SL
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I think it would be unusual (though possible) for the master cyl. you replaced a year ago to have failed. When you bleed your brakes, are you getting clear fluid at a strong flow through all four bleeders? Are you confident your bleeding procedure is clearing air from the lines and not reintroducing it? Is your fluid level dropping, possibly into the vacuum unit? Do you know if the fluid has been changed on a regular interval? My own '88 hadn't had its fluid changed frequently enough, resulting in a failed ABS unit. In my case that was evidenced by a poor, intermitent flow to the rear calipers. Just a few thoughts.
 

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1988 560SL, 2005 BMW X3
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Discussion Starter · #9 ·
Well, I bled the brakes again yesterday morning. Had my little brother help me out and it became substantially better, although, not what it should be so I took it to the shop. They found a leak from the right front brake hose to the caliper. The threads on the caliper are almost stripped. I guess this is causing air to come into the lines. Ordered a new one and should be here Tuesday and will take it back to the shop. This should fix the problem.
 

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82 380SL 96 SL500 03 SL500
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Pump bleeding these cars is about the worst thing you can do with the old Master cylinders because it uses the 70% or so travel thats never used and is always rusted or gunked up, best way is to pressure bleed it or gravity feed it.
I'd never thought of it, but it makes sense.
 

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did the caliper solve the problems???

Well, I bled the brakes again yesterday morning. Had my little brother help me out and it became substantially better, although, not what it should be so I took it to the shop. They found a leak from the right front brake hose to the caliper. The threads on the caliper are almost stripped. I guess this is causing air to come into the lines. Ordered a new one and should be here Tuesday and will take it back to the shop. This should fix the problem.
hi, did the caliper leak solve the issue?...
 

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1980 450 SL
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Make sure you have the bleed screws at top of calipers. I had the rear calipers off my 450 SL to rebuild and put them back on the wrong side causing bleed screws to be on bottom. Could get not too bad pedal from bleeding but as soon as car was started pedal went to floor. Switched calipers back and was able to bleed right away and everything working fine. Discovered my mistake by reading article on brakes in May 2012 Hot Rod magazine. Impossible to bleed brakes with bleed screw on the bottom!
 
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