77 R100RS Rear MC Fluid Level Switch

[kingcowie]

Morning,
I have been refurbing my rear brakes and came across the MC Reservoir Level Switch. This is a normal open switch and i guess it works by it being closed when the brake fluid level comes in contact with the switch. But the max level for the reservoir is below the switch so i am confused how this would work.

I have checked the wiring diagram and it brings on my dash light when it closes.

Can anyone help me out with our explanation.

Thanks kingcowie

[schrader7032]

Rear brake disc or drum? The position of the reservoir suggests rear disc.

My /7 only has the under tank reservoir. I would think that the rear reservoir would work the same in that the fluid would raise up a float that was inside the reservoir, thus opening the circuit to the light and turning it off. If the fluid drops down, the float comes down, and thus closes the circuit, lighting the light.

Is there anything inside your reservoir when you take the cap off?

[kingcowie]

This a rear disc model.
There is no float to activate the switch.
Just the two contacts


i will get a picture today.

[schrader7032]

OK, I looked at some parts online such as at EME. It does appear there is no float. I'm wondering if there is a bellows or something in the cap. Once the fluid level drops in the reservoir, the bellow extends from the inside of the cap to take up the lost volume. In doing that, it trips a circuit through the wires that come into the top of the cap.

[kingcowie]

This is the top and a removed pin.

[kingcowie]

Also i took both pins off and you can see through the other side, ie no pressure switch and also their is a hole through the top as well. ???

[schrader7032]

I guess I'm clueless...doesn't make much sense does it!

I'll just say that the undertank reservoirs had to have an electrical circuit to it because, well, it was covered by the tank and not readily visible. Later when the handlebar reservoir came into use, no electrical circuit because it was easy to look down and see what was there. I suspect the same thing with your rear brake reservoir...it's readily visible so it doesn't need any wires to it. It has outlets on the bottom to feed the master cylinder. That seems to be consistent with replacement parts shown on the EME website.

[srankin]

There is a level sensing system on this reservoir, and yes it is connected to the electrical system. The two spade connectors on the cap are part of the level sensing system.

You are missing the float inside the cap, At least, I don't see it in your pictures. No float, the system will not work as it should.

[srankin]

Sorry, I cut myself off. The picture is from my 78 RS, EME would most likely not have this part as part of their stock. This is most likely a BMW only piece.

The float drops with the drop in fluid, closing the circuit to turn on the bulb in the dash.

There is also a float in the front under the gas tank master cylinder. or should be one, it works the same way.

If the light has been out all this time, whomever removed the float may have bypassed the circuit to keep the light from coming on. I am guessing at all this, as I don't see where you took the float off your self.

A look at the BMW parts microfiche at Max BMW or one of the online BMW shops will tell you what parts are missing. Opps, I didn't look at my cap to see if it had a hole in the top. I am pretty sure it doesn't because a hole would allow moisture or water to get into the system and brake fluid and water don't mix.

Good luck with the repair, St.

[kingcowie]

Srankin,
Now it makes sense, thank you for the assistance.

This bike was a barn build that had some work done to it but not finished prior to me taking it on.
Thinking about it the rear MC also had a missing seal and washer.

Much appreciate your assistance.

Regards Shane

PS this is her in current pull down.