Rusted Brake Line to Rear Wheels

My '96 RMW brake line failed under the driver's door like all GM cars do after they get old--at least in our climate. I replaced the line, bled the rear brakes, but the red light is still on and I have very low pedal.

Do I have to bleed the fronts, too? Is there a path for brake fluid to travel between the two systems? I noticed when bleeding, the level went down in both cylinder reservoirs.

It was amazing that the bleeder screws worked. Usually, in Wisconsin, they're rusted so tight that penetrating oil and/or heat won't free them.

I went through this exact same problem after changing all the lines on my wagon. Bleed all the breaks thoroughly. Rent or buy a vacuum bleeder if you don't already have one. The light comes on when there's a significant pressure differential between the front and rear brake line network or when the ebrake is pulled. If the light is still on after the system is fully bled and you have good pedal feel, do a reverse stop. Get going at a decent rate in reverse and brake hard. If it doesn't work the first time, try a couple more. Doing this will adjust the rear shoes provided the adjuster isn't seized. Last but not least (and this was what caused my problem) make sure the ebrake cables are good and adjusted properly. If not the pedal slop will light up the idiot light.

Thank you. I guess the answer is that there IS a connection hydraulically between the front a rear hydraulic systems.

The connection is in the proportioning valve. There is a piston that can get stuck, and port fluid only to the back brakes. I had to replace one, and the only ones available were used, to the best of my knowledge.

bocoogto wrote:

Thank you.  I guess the answer is that there IS a connection hydraulically between the front a rear hydraulic systems.  

There's two possible problems.

First, as Fred alludes, there is a piston between the two systems, however there is no fluid connection between them. If a line blows, pressure from the unaffected system forces the piston towards the leaking side, actuating the brake light warning by grounding the lead. Unfortunately, this increases the volume in the good side, so the pedal tends to travel further. In theory, once the leak is repaired the pressures equalize and the piston can return. Sometimes that doesn't happen because the piston fails to return.

In that case, remove one front wheel and one rear wheel. Retract the front wheel caliper piston. Remove the rear brake drum and apply a gentle 1/2 pedal stroke followed by a fast pedal return. The front circuit will not build up pressure (because it's taking up the slack). The rear wheel cylinder pistons will extend further than normal against the spring pressure. The sudden return on pedal release MAY be enough to unstick the proportioning valve piston.

The second possibility is that you got air in the ABS system and have to bleed it too. Yes, you bled the wheel cylinder, but there may be a bubble trapped in the ABS body if you did not bleed that too.

And yes, you also have to bleed the front brakes.
Pete

I let the car set for a few days without doing anything. When I started the engine, no warning light, good, hard pedal, and the brakes work fine!! It healed itself!!

Apparently, the switch reset from temperature/pressure changes just with the car sitting.

Problem solved!! Thanks.