Hey Bricksters,

I just finished replacing the brakes on my 1983 242ti. Everything when very smoothly (well almost everything). After we finished bleeding the brakes (the brake pedal seemed normal) I lowered the car and put the wheels on. But as soon as I started the car up the brake failure light stayed on. The brake pedal seems OK (not perfect) and we can't find any leaks. I just replaced the lines a few years back and they are all in good condition. What would cause the brake failure light to turn on. Is there something we could have done wrong during the bleeding process? And how do I reset this light (Haynes doesn't offer much help in this area)? We also heard a small metallic sound from around the brake pedal / master cylinder area when we were bleeding the brakes? Could this sound be someting breaking (only heard it once). Any help would be greatly appreciated (this is my main means of transportation ... it looks like I'll be biking 40 kms to work tomorrow).

Thanks,
Greg

As you can see, there are two schools of thought on this touchy subject - those who feel that, on an old car such as this, the finicky switch is more trouble than it's worth, and those who fanatically endorse obeying the light no matter what.

I'm in the middle - I've found this switch to be far more sensitive than necessary a number of times. I've had it come on after bleeding, and, after a thorough investigation, found it to be giving me a false signal. Sometimes it goes out on it's own. Once, I even replaced the switch after I found it contaminated with crud and water. However, I believe it is a good feature to have, and I take the steps necessary to fix it if it's giving me a false reading.

For example, I just replaced my master cylinder this past weekend. I had gotten a flickering, half-on brake failure light for a week or two, but the pedal felt fine. I immediately suspected the switch was tripping for no good reason. But, last week I stopped at a long light, and, to my surprise, the pedal was oh-so-slowly sinking. I really did have a problem! I replaced the master cylinder, bled the system, and I now have a rock-solid pedal again... but the brake failure light is on full brightness. A few taps on the brake distribution block can sometimes trip it off, but no dice this time. I need to open it up.

If you can rule out all other possible causes of a brake problem, open the switch up. Pull of the wire, and remove the a nylon body, threaded into the distribution block. Be careful, as there is a little plunger and spring inside, and maybe a little collar for the plunger too. Make sure this whole assembly is clean. Spray a bit of brake cleaner up in the hole and let it dry thoroughly. Then reassemble, reconnect, and see if the light went out. This has never failed to put the light out for me, either immediately, or within the first two minutes of running. Of course, if you really do have a brake problem, it will come back on.

Sorry to be so long-winded. Hope this helps...
--
Speed Racer, '83 240 R, '74 164 E, '93 940 OL1 (Manchester, CT)

There could be a couple of probs. It's as Jesse suggests an unbalance or it's the master cyl.

If the pedal drops to the floor on steady gentle pressure then it's the master cyl. If the pedal remains firm then it's an unbalance.

The dist. block is a pain to access. It's on the frame under the brake booster area.

(will check something and get back).
--
1980 245 Canadian B21A with SU carb and M46 trans

Okay, Gt6, has posted since, good info.

Anyhow, RStarkie posted a pic of the distribution/junction block guts on sun jun 15. However, I'm unable to find it doing a search. But it gives an idea of how it works.

Perhaps, someone else can post it.

--
1980 245 Canadian B21A with SU carb and M46 trans

I am not telling anyone to ignore or bypass critical warning lights like the brake failure - merely relating my experience. Both my "Brake Warning" light episodes on the 83 - many years apart - occurred simultaneously with the pedal going soft and giving much reduced braking action.

In other words the light only gave me the same warning my right foot and puckered butt did - Brakes Nearly Gone. Both cases involved a failed (gen. Volvo ATE) master cylinder.
--
Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD)

This happened recently to me when I forgot to check the fluid level in my EZ-Bleeder. I got some air in the Rt front lines. I was lucky. I bled those lines again and the light reset itself next time I stepped on the brakes.

I had the same thing happen. The brakes feel fine. I just disconnected the warning light wire from the junction box. I know the purists might not think that is a good idea but with a car of my age there is risk and reward. Do I want to risk trying to fix the light properly? The reward is not there for me.
--
Simon 80 240 285k 17 years. Not for the longevity or the safety but the heart stopping acceleration.

The brake failure light is triggered by a balance valve between the 2 brake circuits. If there is a significant difference in pressure between the 2 the light comes on. You probably tripped it while you were bleeding the brakes. The bad part is that it will not reset on its own. You have to take the plunger/switch out of the distribution block to let the valve rebalance. It is a threaded plastic plug on the block with a wire on it. The challenge is to get it out and put it back in without destroying it. also keep track of the parts when you take the plug out.
As for driving it, if you have a firm pedal I would not worry about the light. It does not really tell you that much anyway. It comes on at the same time the pedal sinks to the floor, so its kind of redundant and a pain to reset.

Almost certainly what you have done is cause the master cylinder to develop an internal leak, probably by over-enthusiastic bleeding. I know, because I have done it myself before I learned NEVER to pump the pedal when bleeding further than it travels in normal use. This runs the seals over unused sections of the bore, which may be dirty or corroded. Once the rubber seals have been damaged the internal leakage means the two circuits may have different pressures.
You need a new MC. The light will probably cancel itself (dismantling not recommended, and usually unnecessary). Don't ignore this - brakes are a matter of life and death, and with only half the system working you have already lost one of your two lives.