5 ton master cylinder question

[bigmike]

I replaced my master cylinder Saturday. After removing the old one, I found this thingy (picture attached and I'm pointing at it) on the bottom of the unit. The new master cyllinder has the same port but it is plugged. There were no wires attached when I pulled it out but it does take wires.

Any ideas? The truck runs fine without it hooked up...I guess...not really sure because I just bought the truck...

BTW, I didn't see anything in the TMs when I reviewed them during the master cylinder replacement process.

 

[cbvet]

It's obviously a fluid pressure operated brake light switch.
Unless there is one hooked up somewhere else, you have no brake lights!

 

[nk14zp]

Brake light switch. Your truck might have the MWO for the air opperated brake light switch.

 

[bigmike]

I have brake lites. I checked them after replacing the M/C.

 

[M543A2]

The five tons we have all have a switch that triggers the brake lights in an air line at the booster, not the master cylinder. When the air pressure goes to the booster to apply the brakes, it triggers the switch. It is inside the passenger side frame rail behind the master cylinder. It has two wires attached to it. Of course, you can test the brake lights by having someone check them when you push the brake pedal with air pressure up.
I would say someone had put a master on the truck off another application that needed the wires, and did not hook them up on your truck because they were not needed.
As CBvet said, the mystery switch is for trucks that do not have the brake light switch on the air line. Our trucks are not the newer 8xx series, so I do not know how they triggered the brake lights on the newer Cummins series trucks.
Regards Marti

 

[M543A2]

CORRECTION!!!
The air pressure brake light switch is inside the driver's side frame rail, NOT the passenger side!

Regards Marti

 

[bigmike]

thanks guys. this truck is a M818. there is a wire hanging down right where the M/C is. I'm still not 100% sure what was going on but everything works so I'll not worry about it.

 

[KsM715]

Dont worry about it. There's always supposed to be left over parts/wires when we do little maint. projects.

 

[akonitony]

Why that's the biggest... one of those I've ever seen!

 

[nep]

brake pedal stiffens up after driving for 10 miles or so. new booster ,master cylinder , lines. Oh yeh this is a m818. Can any one help