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7 series brake master cylinder. Do it!

Posted by nomis3613 
January 09, 2010 04:28PM
To get rid of the mushy brake pedal feel, yesterday I upgraded the master cylinder to a 25mm model from a E32 750i. The part bolts straight in (although a T-piece is needed if you don't have an ABS car) and the result is amazing!!! The pedal is now rock solid, with great feedback.

A great improvement for our cars, just wanted to pass on the news.
rkj
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nomis3613
To get rid of the mushy brake pedal feel, yesterday I upgraded the master cylinder to a 25mm model from a E32 750i. The part bolts straight in (although a T-piece is needed if you don't have an ABS car) and the result is amazing!!! The pedal is now rock solid, with great feedback.

A great improvement for our cars, just wanted to pass on the news.

Sounds like a good upgrade, anytime you increase bore size everything works better/easier thumbs up thanks for the tip, nomis? Is that your name???

Rick
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rkj
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nomis3613
To get rid of the mushy brake pedal feel, yesterday I upgraded the master cylinder to a 25mm model from a E32 750i. The part bolts straight in (although a T-piece is needed if you don't have an ABS car) and the result is amazing!!! The pedal is now rock solid, with great feedback.

A great improvement for our cars, just wanted to pass on the news.

Sounds like a good upgrade, anytime you increase bore size everything works better/easier thumbs up thanks for the tip, nomis? Is that your name???

Rick

Think back to front winking smiley

A bigger bore isn't the secret here...a smaller bore yields more mechanical advantage. therefore the E32 cylinder is a much more efficient one.

I don't mind the mushy feel because my iX has extremely powerful stopping power compared to most other cars I have driven smileys with beer
rkj
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Archeo-peteriX
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rkj
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nomis3613
To get rid of the mushy brake pedal feel, yesterday I upgraded the master cylinder to a 25mm model from a E32 750i. The part bolts straight in (although a T-piece is needed if you don't have an ABS car) and the result is amazing!!! The pedal is now rock solid, with great feedback.

A great improvement for our cars, just wanted to pass on the news.

Sounds like a good upgrade, anytime you increase bore size everything works better/easier thumbs up thanks for the tip, nomis? Is that your name???

Rick

Think back to front winking smiley

A bigger bore isn't the secret here...a smaller bore yields more mechanical advantage. therefore the E32 cylinder is a much more efficient one.

I don't mind the mushy feel because my iX has extremely powerful stopping power compared to most other cars I have driven smileys with beer

Huh, I wouldn't have thought that. I know in motorcycles a larger bore master (at the bars for the front brakes) is supposed to help with lighter lever action and more stopping power. That's what I read anyway confused smiley
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Archeo-peteriX
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rkj
thanks for the tip, nomis? Is that your name???

Rick

Think back to front winking smiley
That's it!! :wink:

The E32 bore is bigger, so you're pushing more fluid for a given distance > more braking force and a "harder" pedal.

Like Peter implies, it doesn't actually increase you're braking power, just the pedal feel (which is a personal preference). Now that I'm enjoying the feel of the brakes so much, upgraded pads probably aren't far away...

Simon
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rkj
Huh, I wouldn't have thought that. I know in motorcycles a larger bore master (at the bars for the front brakes) is supposed to help with lighter lever action and more stopping power. That's what I read anyway confused smiley

Yeah, I'm not 100% sure how it helps, either. Maybe it's different because our brakes are vacuum assisted and the master cylinder is downstream of the vacuum booster.
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rkj
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Archeo-peteriX
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rkj
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nomis3613
To get rid of the mushy brake pedal feel, yesterday I upgraded the master cylinder to a 25mm model from a E32 750i. The part bolts straight in (although a T-piece is needed if you don't have an ABS car) and the result is amazing!!! The pedal is now rock solid, with great feedback.

A great improvement for our cars, just wanted to pass on the news.

Sounds like a good upgrade, anytime you increase bore size everything works better/easier thumbs up thanks for the tip, nomis? Is that your name???

Rick

Think back to front winking smiley

The dynamics of mechanical advantage are that the smaller bore at the pressure applying end; the more advantage at the caliper end. Think of it like sprockets and gearing; the smaller the drive sprocket, the more torque.

A bigger bore isn't the secret here...a smaller bore yields more mechanical advantage. therefore the E32 cylinder is a much more efficient one.

I don't mind the mushy feel because my iX has extremely powerful stopping power compared to most other cars I have driven smileys with beer

Huh, I wouldn't have thought that. I know in motorcycles a larger bore master (at the bars for the front brakes) is supposed to help with lighter lever action and more stopping power. That's what I read anyway confused smiley

I guess you read wrong sad smiley
If the bore at the handle bars is larger, the mechanical advantage is lost along with the torque. The pedal may feel harder but the braking effort at the pedal will be higher.

Think about sprockets...the smaller the drive sprocket, the more torque you get at the driven one. Same goes for brake cylinders.
I know it sounds counter intuitive but thats physics for you.

There must be something else going on in the E32 master cylinder if the brake pedal is harder and the braking is more powerful.

There is something else going on with the
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nomis3613
The E32 bore is bigger, so you're pushing more fluid for a given distance > more braking force and a "harder" pedal.
Ideally it would be nice if there was no distance involved at all. Pedal travel is just whatever it takes to get the brake pads seated firmly against the brake rotors. After that, the only thing that matters is how much force is being applied to the pedal, which generates a corresponding pressure in the brake fluid, which then ultimately applies a force against the brake pads.

If you haven't changed anything else downstream, only the bore size of the master cylinder, then it will still require the same fluid pressure as before within the brake lines to generate the same braking force at the wheels. However the pedal-feel will change.

A larger bore master cylinder will have a bigger piston with a larger surface area. In order to generate the same pounds per square inch brake pressure within the brake lines, you will need to apply more pounds force to the brake pedal as that force is now being distributed over a larger square inch surface area on the master cylinder piston face.

lbs / sq in

Yes, the bigger piston will move more fluid for a given stroke distance. So the pads will seat against the rotors with less pedal travel. But it will require more force on the brake pedal than before to achieve the same amount of braking effect at the wheels.
rkj
Thanks Peter and Ferdinand, that's what I like about this joint.....you learn smiling smiley
February 18, 2010 12:32AM
Yap, we r laernerz spinning smiley sticking its tongue out
Aloha everyone
February 18, 2010 04:14AM
G'day!
I thought the mushiness was more a problem for us RHD drivers, but it sounds like LHD have it too.

For those who don't know, on RHD E30s the booster and MC are on our passenger side. A large tube takes the action from the brake pedal, under the dash, and then into the engine compartment from behind the glovebox. Not very efficient!
Supposedly you can adjust it but haven't tried that yet.

What T piece do we need for non-ABS ?
I don't know exactly (mine is ABS), but basically it's a custom made splitter to send the fluid to the separate front and rear circuits on non-ABS cars. Apparently you can get a brake shop to make one up pretty cheap.

Useless E30 trivia: the RHD models have separate clutch and brake reservoirs cos the brake master cylinder is on the other side of the engine bay
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