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  #1  
Old 04-11-2013, 08:18 AM
JamesDean's Avatar
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Brake Master Cylinder Pressure/Specs

Hey everyone,

On the bottom of most of our brake master cylinders there are two numbers, I think they typically look something like "19/25", maybe different numbers based on model..

I've read and been told those numbers indicate the pressures at which the front/rear of the master cylinder operate? Or am I understanding that wrong?

Does anyone have a chart comparing different models of brake master cylinders's pressure markings? I.E 190E 2.6 master vs 420SEL vs E420 vs E500?
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  #2  
Old 04-13-2013, 05:41 PM
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Answer

Those are internal manufacturing design engineering codes.

As far as I am aware, the data is proprietary (trade secret).



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  #3  
Old 04-13-2013, 07:49 PM
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Thanks Roy. That kind of what I thought they were originally, but I'd read somewhere that they indicated pressures or something of that sort with respect to the two halves.
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Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

82 300SD 145k
82 300SD 265k
87 420SEL 230k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k
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  #4  
Old 04-13-2013, 08:56 PM
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Some master cylinders have bore sizes cast on the housing. One that is marked 19 / 25 is a stepped bore, 19 mm / 25 mm .

Stepped bores ( along with some internal valving ) are usually used to provide high volume / low pressure to bring caliper pistons out rapidly then switch over to high pressure / low volume for actual braking. If you had a large bore only, the peddle would be very hard, if small bore only peddle travel would be very long. This system is sometimes used on industrial equipment ( like forklifts ) to provide power braking without any " power unit ".
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Old 04-14-2013, 02:18 AM
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The FSM hints at the possibilities!

If you look at chapter 42-015 the high pressure test needs to be between 50 and 90 bar

That's quite a bit => 725 PSI to 1300 (and a bit) PSI

EDIT this is W123 FSM - probably same chapter for other vehicles of a similar age though
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Old 04-14-2013, 02:48 AM
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Quote:
Originally Posted by 97 SL320 View Post
Some master cylinders have bore sizes cast on the housing. One that is marked 19 / 25 is a stepped bore, 19 mm / 25 mm .

Stepped bores ( along with some internal valving ) are usually used to provide high volume / low pressure to bring caliper pistons out rapidly then switch over to high pressure / low volume for actual braking. If you had a large bore only, the peddle would be very hard, if small bore only peddle travel would be very long. This system is sometimes used on industrial equipment ( like forklifts ) to provide power braking without any " power unit ".
Using the lower limit data in the FSM (see post above) and a bore diameter of 19mm I reckon the force required is =>

Force = Pressure X area

area = 2.83e-4 m^2

Pressure = 50 bar = 50e5 Pa

Force = 1417 N ~ 145 kgf ~ 320 lbf

I don't know if that sounds about right - do you?

I realise that this isn't the full story - there's a brake booster - and then the mechanical advantage of the brake pedal =>

BRAKE MATH: CALCULATING THE FORCE NEEDED TO STOP A CAR: Brake and Front End

More data needs to be found - if this is of any use to the OP!
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Don't leave that there - I'll take it to bits!
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