"brake failure" light and red hot brakes (308)

stuck caliper???

 

I did experience this once and the problem was one of my rear piston in the caliper was stuck and reason for being stuck was that when I opened the caliper to rebuild it I found the inside of the caliper was coroded and that the o-ring or the seal around the piston was in bad condition. So after a I cleaned the caliper and installed new o-ring and flush all the brake lines the problem never repeated, it's been 6 years now.

So you may want to start by just flushing and bleeding your brake system and check the mechanical parts that controlsthe parking brake make sure nothing is binding.

 

Yep yep.

 

unlikely.........

Two things come to mind that I have seen.

One is a stuck check valve in the master cylinder; the other is a swollen rubber brake line.

When they get old the brake hoses swell internally and act just like a check valve holding pressure in the caliper. Its not a great deal of pressure at first and you can still drive, but as the brakes heat up due to constant friction the fluid expands creating enough pressure to lock the disc. I have seen this happen so bad that the pad backing plates looked like a potato chip. That was in a Porsche. I have seen Mercedes lock up to the point they had to be towed to my shop.

Interestingly I have never seen this happen with the front brakes, its always been rears that locked.

An easy diagnose is to see if you can retract the pistons in the rear calipers. If you can’t, disconnect the hose where it connects to the hard line (do not disconnect it from the caliper) and try to retract the pistons. If they retract, the problem is the master cylinder, if they don’t it’s the hoses. Take the hose off and try to blow through it to prove it.

 

The way that the brake failure light works is that there is a pressure differential switch that turns the light on if there is a difference in pressure between the two brake circuits. A stuck caliper or bad brake hose will not cause the light to come on. Only the bad master cylinder will cause this problem.
I would recommend rebuild/replacement of the master cylinder.

 

Not just a bad master cylinder. I had this light come on when tracking my old 308. It doesn't take much in that car to "boil the brakes" - three laps by an amateur (i.e. brake-heavy!) driver, such as myself, and my brakes were cooked. The brake fluid was literally boiling since it had been heated past its boiling point as a result of heat transfer to the fluid from the brake discs and calipers.

So one theory that fits the data is the a caliper got stuck and you boiled the brake fluid, causing a loss in braking pressure, which was detected by the pressure switch which activated the brake failure light. You should have detected a sponginess in the brake pedal.

If the brake fluid was old, it could have absorbed water and been more prone to this as the brake fluid boiling point reduces as it absorbs water with age.

One of the first things I would do if an otherwise untouched 308 came into my hands is to rebuild all of the brake calipers. I had two on my own 308 just start leaking on their own.

 

Right. You boiled the brakes. There is air in one of the two hydraulic brake circuits (left front connects to right rear, this is one circuit; the other diagonal is the other circuit).

Solution is to "bleed the brakes"--purge the brake circuits of air. At this point, frankly, you should probably also change your brake fluid if you're going to do a thorough bleed (fluid cost is < $20).

However, whatever caused this problem may come back. You need to find the root cause, fix it, then do a brake fluid flush and bleed.

T. Rutlands has rebuild calipers via a core exchange which I was pretty happy with. But it can be done cheaper if you do it yourself (I wasn't to the point then when I felt comfortable rebuilding my own brake calipers).

 

If the emergency brakes were adjusted recently, they may be adjusted a little too close. Some regular braking will heat up the disk, thus making it contact the emergency brake pads, creating more heat, etc.

This happened on my 330 once. One rear side red hot with some excess grease burning. You should have seen me blowing through the holes in the alloy wheels while yelling at my passenger to get out the fire extinguisher.

 

The reason that I think that it is the brake master cylinder is because of the sequence of the symptoms. The brake warning light came on first, then the brakes felt like they grabbed. The feel (mushy) of the brake pedal did not change until the next day.

If it was a stuck caliper or bad hose that caused the brake fluid to boil and change the pressure in the system, then pedal feel would have changed drastically before the light came on. But this is not the case, the first thing that happened was the light coming on.

I stand by my recommendation of having the brake master replaced. While you are working on the brakes, the seals on the caliper that got hot should be replaced and all the other calipers should be checked for stuck pistons and hardened seals/dust boots. You can check the hoses for flow when you bleed the system, but if they are old, now is a good time to replace them. The rear brake pads should be replaced as well.