1977 450 SEL oddball mc

[Russgary]

I am getting ready to dig into a brake rebuild on my 1977 Mercedes 450 sel, as I'm collecting parts it became obvious that I have an "oddball" master cylinder.  The type that looks just like the 60 Dollar one, but has a switch and costs 350.

Does any member have experience with the only difference between the 2 is the switch.  I can't imagine this is the switch that operates the brake lights, more likely a differential switch, between the front  and rear circuit.

[daantjie]

If you look at the EPC it does appear to show 2x master cylinders, which maybe means a VIN break took place?  Not sure, but here is the link to the EPC I use:

https://partsouq.com/en/catalog/genuine/unit?c=MB201810&ssd=%24%2AKwHe6vvO1Jqdkp-YmaaDj4aSsrWr2tXY2cvk15-Zqr6ipqSn4vXZs52Znay6o7L06ePe197e3M25rfmb8r-slIislsicl4GFqqrIx86AyNLbzsuAwciMztfY0YWJnpGTmoys3sXdy87nl8-eydbPqKWXl4WFz9DY3N3S2ZmZwM7X29_U1NzeyMXRiYWqkM6byNKXmYvOgInJ1s_b2YOLlN_PxsmfjsjSl5mLzoCBydbPrMqZmcSCz9DJ2tWkypmZ0Y_P0Mndz5cAAAAABlZKZA%3D%3D%24&vid=771&cid=29029&uid=30081&q=

Maybe a more learned member will chime in

[daantjie]

Further, I looked at the 6.9 model brake master cylinder, and this one pops up as the superceded part number 0034305901 referenced, which looks like the one you show:

https://www.autohausaz.com/pn/0034305901?srsltid=AfmBOoqjR3aPXWbykJtx01R4FGAk2j5g-H5aBw-cwx5cN1O9wUG8t3i2

Earlier number is 0044300901:

https://www.autohausaz.com/pn/0044300901?srsltid=AfmBOoqFu3OozzNNUCM-h1OLjfjBA7Ui3FvMzWh0OUqFONjcJMozxFir

Also, that switch is not for the brake lights but rather a pressure sensor. Likely you can just fit the cheaper version without the switch I would think.  I am pretty sure the pressure switch lights up a warning light in the instrument cluster if you're low on fluid and such.

[rumb]

I take it that your car has one with the switch.  Ground the wire going to it and see if your brake lights turn on.  If so then that what you need.

Sometimes you just have to pay the piper.

This might work

https://mercedes-classic-parts.com/product/hoofdremcilinder-v-nr-0024302801-repro/

https://www.ebay.com/itm/156192876894

[Max-NL]

According to the factory manual it's a pressure differential switch, which warns the driver if there is a pressure difference between the front and rear circuit.

See attached PDF. It seems that is a simple grounding switch which is pushed in by the rod which is connected between the front and rear pistons. If pressure is lost in either front or rear the rod is pushed and the switch is pushed in.

Either disconnect the switch or ground it (not sure about it) and you should be able to use a regular one.

[rumb]

That is very similar in design to a unit on MG's of same era. Fairly worthless in my opinion.  Certainly not necessary if you just want to get the less $$ unit,

[UTn_boy]

The switch is actually a mechanical switch rather than a pressure switch.  It alludes me as to why it's referred to as a pressure differential device.   It works like this....

If the brake pedal were to go to the floor or further down than it's supposed to due to wear, air in the system, or loss of fluid, the internal piston of the master cylinder will make contact with the switch pushing it "on", so to speak, lighting up the "brake" light in the instrument cluster....I think it's the same light that comes on when the parking brake is on.  It doesn't spring back.  After repairs are made, you'd simply push in the center of the switch to reset it.

Back when I had my 280S, it, too, had this expensive master cylinder.  I couldn't afford a new one at the time, so I bought a used one off of a 1979 450sel in the junk yard.  It was the later one without the switch.  I took a chance on it since it was my car and not someone else's.  It ended up working just fine, and has been on there for 11 years now.  So theoretically, you could buy a new one for the later cars, and would work just fine, but the warning light wouldn't work anymore.  Then again, if the pedal goes down far enough to trip this switch, you'd know it anyway....with or without the switch being in situ.  But if you're really into originality and keeping things as they were, then like RUMB said you'll just have to pay the piper. 

[Russgary]

Im having difficulty posting.   But I got the correct master cylinder ( New old stock) for roughly 150 dollars. My reservoir is in excellent condition.
For better or worse I'm using Brand new Asian (I'm assuming) replica calipers, new hoses, and new rotors.

[Russgary]

According to the factory manual it's a pressure differential switch, which warns the driver if there is a pressure difference between the front and rear circuit.

See attached PDF. It seems that is a simple grounding switch which is pushed in by the rod which is connected between the front and rear pistons. If pressure is lost in either front or rear the rod is pushed and the switch is pushed in.

Either disconnect the switch or ground it (not sure about it) and you should be able to use a regular one.

Awesome info.  An electrical method for indicating an otherwise obvious event. No warning light needed.