I had a problem with the brake light staying on due to "hand brake to master cylinder push rod" not making contact.
OK, let's back up. I have an '02 A/C 500i. It has about 2400 miles on it now but I can't be sure due to past problems with odometers, broken speedometer cable, etc. Give or take 100 miles.
The '02 model has the aluminum brake master cylinder, which is the prefered style as opposed to the plastic style. Sometimes, when parked or during use, the piston inside the master cylinder, won't return to the proper dimention which would make it in contact with the hand brake and thereby turn off the brake light. I didn't pay this much attention in the past but yesterday, I was going riding when I noticed the piston had not returned at all. It was fully depressed but there was no brake pressure on the rotors. The rotors would essentially free spin. The hand brake had no effect when pulled against the piton rod of the master cylinder. The first thing I thought was to take the cover off to see if there was any fluid in the reservoir of the master cylinder. It was full so now it was time to break the line and see if fluid was flowing. There was a drip out of the master cylinder but the line had no back pressure. I removed the master cylinder from the handlebars and called a mechanic friend who works on quads often. I wanted to know how the piston and cylinder are to be disassembled but he hadn't the knowledge of A/C's units.
I had to figure out what was happening for myself. On the bench, in the garage, I placed the master cylinder in the vise, careful not to stress weak points. I pulled back on the piston rod until the indentation was showing through the bottom, rear hole, in the reservoir. Then I used 90# of compressed air to blow both ways; through the line side then back through the reservoir. Now with the piston moving freely, I put all back together, added new fluid, bleed the brakes and it worked fine the rest of the day.
I think I'm going to have to find instructions for teardown & rebuild of the master cylinder before this happens again. I don't know what was stuck in the cylinder. Never saw anything but it's obvious, there was something blocking the fluid from flowing. If any of you have information, literature about these master cylinders, don't forget to share it with us.
Chuck

 

I also had a '02 Cat and had the same trouble. I believe you fixed your problem when you bled your brakes. I had mine replaced under warranty once becuse they were a poor design and would get air in the lines over time which would cause the piston to not spring back properly (because of the lack of back pressure in the lines). The new one did the same thing. I just lived with bleeding my brakes every so often and it worked fine. I'd be willing to bet that you notice that your brake lever is now harder to pull all the way back (less spongy) and stops you better than it did before you did the tear down and bleed. Mine would get so bad that the lever would touch the handgrip on a hard stop. Bleeding worked like a charm!

Glad you got it working!

 

Got the same problem with my 02 500i, but so far I havent noticed any sponginess in the brakes. The master cylinder piston is coming almost all the way out, but not completely.

 

ethernut, you may have a good point about the air in the line. But my brakes never felt spongy. The piston rod just didn't come back all the way out. I can only think of three reasons that would happen:
1. Debris in the cylinder at the piston wall.
2. Broken return spring.
3. Blockage in the airway causing lack of atmospheric pressure.

I noticed today when I was washing the AC, the brake light is on again. The piston does return. Just not all the way. I'll see if my dealer has a clue, what's going on.
John, I think you and I have the same thing going on. if you come up with a cure, let me know.
Thanks guys,
Chuck

 

Replacing my master cylinder fixed the issue with the piston. Fortunately they did it under warranty. But as much riding as I did, I found my self bleeding the new master cylinder every 300-500 miles.

Just for grins, try re bleeding brakes to see if there is air in the lines. If so, you may be experiencing that faulty master cylinder issue I experienced. I'd get TINY bubbles that almost seemed impossible to get out.

 

OK, ethernut, I will try it and post the results. See ya tomorrow, here.
Chuck

 

had the brake light problem with a 98 454, but it wasn't the master cylinder on this one. The push button switch was loose and didn't make good contact when the the lever was all the way forward. Drilled it out some at the point of contact, and JB welded a small nut on there to fill up the space. Then grinded it off to fit. Not exactly NASA engineering, but it keeps the brake bulb from burning out from being on at all times.

 

Guess what? That's what my dealer did the first time this happened. And I guess it worked for a while. Looking at the hole and the bolt, neither is worn or out of shape. I think it's really a **** poor design if we have to keep drilling out the old hole. Did you have anymore trouble after?

 

I noticed all the replies are about 2002 models. I recently bought my wife a 2002 375 with 308 miles on it. It had the same problem. I replaced the master cylinder, but we have only put about 150 miles on it. I have a 1999 300 that I bought new and have never had a problem with the brakes on it other than having to replace the pads ever so often. I am wondering if another year master cylinder would not bolt right up and maybe eliminate this problem. Just a thought.

 

Mine had nothing to do with the master cylinder. Had to do with the plastic thing that covers the brake light switch and holds it in place in the brake assembly being loose, and not keeping it tight enough for the lever to depress the switch button.