Brake issues

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Henrik

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My brakes are engaged without touching the brake pedal? As you can see from the picture there is a 2mm gap between the master cylinder and the brake booster. When I close this gap by tightening the two nuts the brakes are engaged. The brake pedal adjustment is according to the book and the brake piston rod is in "neutral" position. The brake booster plunger is also in normal/neutral position as far as I can see, it is not possible to force it further back.
The Brake Master cylinder has new gaskets and piston.
I have measured the distance from the end of the piston to the flange to be 49,25 mm, is this correct?
The brake master cylinder is marked: 23 and 25.,and -6 ATE XX
Another detail I have noticed is that the tip of the brake booster plunger is about 2mm outside the brake booster housing, This is the same as the gap. Is it correct or should it be flush?
 

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JMinPDX

JMinPDX

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I’m no expert and it has been years since I replaced mine but I suspect your master cylinder is the problem. I don’t believe the rod from the brake pedal is adjustable. I thought the adjustment was for pedal height. I could be mistaken but at least I got your question back to the top.
Good Luck
 
Ohmess

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I would think you could get some adjustment on both ends of the piston rod that connects the pedal to the booster. The end under the dash where the rod connects to the fork at the pedal would probably be the easiest place to start. You would likely have to remove the fork from the pedal and spin it on the rod in order to change the length.
 
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Henrik

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I have already made some adjustments to the rod so that it does not press against the brake booster when the pedal is in neutral. I have also observed that the diaphragm is in neutral position (i.e. no forces affect it from either side). My concern is why the brake booster plunger is about 2mm outside the brake booster housing? Or is this normal? If so, why does it push the master cylinder piston in about 2mm when assembled? Could it be that the (new) piston is badly machined?
I can probably solve the problem by drilling another 2mm into the end of the piston to accommodate the whole plunger, but I'd prefer to check out all possible sources of error before doing anything drastic:) BTW the piston sticks 49.25mm outside master cylinder flange and is 49,25mm deep. Can someone confirm if this is correct?
 
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Krzysztof

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Henrik said:
My brakes are engaged without touching the brake pedal? As you can see from the picture there is a 2mm gap between the master cylinder and the brake booster. When I close this gap by tightening the two nuts the brakes are engaged. The brake pedal adjustment is according to the book and the brake piston rod is in "neutral" position. The brake booster plunger is also in normal/neutral position as far as I can see, it is not possible to force it further back.
The Brake Master cylinder has new gaskets and piston.
I have measured the distance from the end of the piston to the flange to be 49,25 mm, is this correct?
The brake master cylinder is marked: 23 and 25.,and -6 ATE XX
Another detail I have noticed is that the tip of the brake booster plunger is about 2mm outside the brake booster housing, This is the same as the gap. Is it correct or should it be flush?
Click to expand...

I have had a problem with repair kit for the brake pump in other BMW model. After replacement it was not working as it should despite the fact I was sure all was fine.

In the end I have bought new complete as I was not ready to risk more. Despite difficulties to flush the fluid across all the lines it worked perfectly from the beginning.

Maybe check the master cylinder once again or consider changing a complete one. Brakes are one of the main component aren't they?
 
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Henrik

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The Master cylinder is out and I will check the inside today. Don't know what to find or look for but I have to check it off the list!
 
Krzysztof

Krzysztof

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Hope you will nail the problem.

During my repair I was sure all was put into the right direction.
For the pump the most important is the direction of the rubber rings as they are typically non-symmetric.
Maybe the spring was not properly seated (stuck before the end stop)?
Just guessing.
 
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Henrik

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Checked the inside of the Master Cylinder today, all well. Then I smeared some stuff (similar to plastigage) on the tip of the brake booster plunger and mounted the master cylinder. When disassembled again I could see that the plunger did not hit the bottom, but at a different point in the curve in the rounded end inside the piston. So I used a 7,5mm drill bit and changed the curve to be more like the tip of the plunger. This made it possible to get the plunger about 1mm further into the piston. After assembly and brake bleeding wheels can be turned by hand. Problem solved:)
 
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