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1983 300SD (90K), 1985 300CDT (69K) brand new, 1970 250C, 1989 190D (45K) brand new
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164 Posts
Discussion Starter #1
Does anyone have a good way to test the brake booster? And has anyone replaced one on the SD?

I've owned the car for just a year now, but the brakes have always been a little soft. Recently I discovered I had two leaking calipers. So I had all 4 calipers replace along with new rubber brake lines connecting to them and new brake pads. When done, the entire brake system got flush and bled.

Hmmm, brakes a little more mushy than before. I know the standard thing to do is suspect the master cylinder. I thought that initially. However, the brakes ALWAYS work great the very first time you press the pedal. More than one press of the pedal and the braking ability degrades rapidly. That indicates vacuum issues. Anyone who has had a call stall on them while pressing the brake pedal would know what that feeling is like.

After testing various vacuum lines, I have determined that the Vacuum pump itself is pulling 20 inches. With everything connected, it pulls between 10-15 inches. When I step on the brake more than once, it goes nearly to zero.

If I disconnect the Vacuum line directly from the brake booster, the entre system (minus the booster) is pulling 20 inches. That tells me the booster is leaking profusely.

If I missed something, let me know. I want to be ABSOLUTELY sure the booster has gone south before I replace it. They cost $588 for the part alone. Would anyone even suggest getting one from a junk yard? Any insight is appreciated.
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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10,152 Posts
I understand to pull vacuum on the booster itself, and see if it holds. I imagine you'll need a series of adapters to hook up to a mity-vac.
 

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83 500SEC, 84 300TD, 86 190E 5.0 M113, 87 300D, 01 CLK55, 02 C32, 02 C320T, 03 C320T4, 03 E55
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2,600 Posts
when my booster was bad I had one pump then an extremely hard pedal, not mushy as you describe. I would definitly get one from a junkyard, thats the route I went.
 

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1983 300SD (90K), 1985 300CDT (69K) brand new, 1970 250C, 1989 190D (45K) brand new
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164 Posts
Discussion Starter #4
I understand to pull vacuum on the booster itself, and see if it holds. I imagine you'll need a series of adapters to hook up to a mity-vac.
Yeah, I just stuck a tube into the opening of booster and started to pump the mity-vac. After 5 min of pumping, I got tired and only got 10 inches vacuum. That booster contains a lot of surface area.

I'm going to call a JY this morning to see what's available.
 

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83 300sd 93 300e Alot Of S-series
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180 Posts
Most sd's i see in the JY are missing the brake booster already ... mine leaks profusely but im sure its the m/c leaking fluid back into the booster not the booster leaking itself
 

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1983 300SD (90K), 1985 300CDT (69K) brand new, 1970 250C, 1989 190D (45K) brand new
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164 Posts
Discussion Starter #6
Yesterday I picked up a booster from a JY.

Anybody know what seals I need?

AutohausAZ shows I need a large tube bushing. Performance Products does not show the tube type bushing. Instead it shows only a small O-ring. So which one is needed?
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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10,152 Posts
I can't find where that bushing goes, but it's cheap and you may need it inside, of course, you may be able to use the old one.
You will need that "O" ring, part #43-010 from performance prods. Goes between the booster and M/C.
 
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