Brake Master Cylinder Pressure/Specs - PeachParts Mercedes-Benz Forum

JamesDean · #1 04-11-2013, 08:18 AM

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?

whunter · #2 04-13-2013, 05:41 PM

Those are internal manufacturing design engineering codes.

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

.

JamesDean · #3 04-13-2013, 07:49 PM

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.

97 SL320 · #4 04-13-2013, 08:56 PM

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 ".

Stretch · #5 04-14-2013, 02:18 AM

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

Stretch · #6 04-14-2013, 02:48 AM

Quote:

Originally Posted by 97 SL320

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!