: Mystery brake warning light

10-02-11, 10:41 AM
I have an '84 Sedan DeVille, 4.1 liter metric engine. The brake light stays on all of the time. I do not have ABS. I disconnected the park brake switch and light stays on. Restored that and disconnected a sensor switch on a brake line junction about a foot below the Master Cyl. The light stays on. Brakes work fine and no leaky cylinders.

Am suspecting that the screw-in sensor switch may have gone open causing light to stay on. This casting that all of the lines come in and out of has two inputs from the mast cyl, and 4 lines to the 4 wheels. The sensor switch screws into the casting between the two lines from the mast cylinder. I suspect it is supposed to detect a difference of pressure from the two chambers of the mast cyl. Owner's manual did not even tell me there was a sensor of any kind. ???

Any Ideas? Anyone know what that gadget is called???


10-02-11, 11:18 AM
The switch may have gone bad.. But I have never seen that happen.. If your a 100% sure that there are no leaks, the only 3 things that it can be is air in the system, a bad rubber hose or the switch ... A hose can swell up on the inside and keep the fluid from flowing back and keeping the brakes on , sometimes a little on , somtimes a lot..

10-02-11, 11:30 AM
The brake light is triggered by one of two things happening. First, the parking brake being set. Second, a pressure differential between the front hydraulic system and the rear. Usually, this is caused by a leak. The proportioning valve is the part that has the electrical connection for this. Inside, there is a little piston that moves to the end of the brake system that has the lower pressure.

So, we need to look at how to fix the light.

First, you must eliminate the possibility that there is actually a leak. I know you said the wheel cylinders are fine, but that is not where leaks normally happen on these cars. The lines rust. Most common is up on top of the driver's side frame rail under the door. Check the level of the brake fluid in the master cylinder. It should be up to the max level, and the rubber gasket should not have the middle parts pulled down into the master. If they are, there is likely a leak.

If there is for sure no leak, there are two possibilities left. First, the piston is displaced, and needs to be reset. Second, the switch is bad. To reset the piston, there are a couple of things that might work. First up: in an empty lot, put the car in reverse. Get it up to about 20, then hit the brakes as hard as you can. Repeat in drive. Try it a few times. This works after the system has had a leak repaired. If that doesn't work, you can try tapping the prop valve with a hammer. If that doesn't work, then you need to look at replacing parts.

csbuckn will probably jump in here. I know that he went through something similar.

10-02-11, 04:26 PM
Thaks DRMenard and Jayoldschool,

I will definitely reset the master reservoir gasket. I had two barely leaking wheel cylinders, both in the back. That would do it I suspect, but it is still lit. I will reset the gasket/diaphragm at the reservoir now and see what happens. Will check all four hoses, but saw no leaks in vicinity when looking at it. I just did replace a leaky wheel bearing/seal at L rear and discovered both wheel cyls in early stage of leaking at that time. Both are rebuilt now and successfully bled them and had good test drive. Will also look closely at the lines for rust pinhole. (Had that in an old Ford truck around 6 yrs ago or more.)

Oh, I have not had any luck finding that switch either, or least NAPA has not got it. No Cadillac dealership listed in phone book in this town of 250,000 since GM became "Govt Motors". Just my luck.

10-02-11, 05:12 PM
Ah, you should have said that you had leaky cylinders in the first post. This is what caused the light. You need to properly bleed the system, then do the stop test. The light will go out. It is still on because you haven't bled properly.

10-02-11, 05:48 PM
No one has mentioned the vacuum switch plugged into the brake booster. If the belt driven vacuum pump is weak or the wiring to this switch is grounded, the light will be on. Jumper the wiring connector to this switch, see if the light goes out, if so either a faulty switch or a weak vacuum pump. Pump should pump a steady 18 to 19 in of vacuum. I own the same car. To center the brake proportioning valve switch you must crack a hydrallic line screwed into it and depress the pedal. That should center the contacts in the switch.

10-22-11, 08:34 AM
Thanks for input carnut and jayoldschool. I have been doing other things lately. Forgot to mention that when I disconnected switch wires, the light did not go out. That was at the time I rebuilt the two rear cylinders. Jay, I did bleed brakes to rear quite a while until I saw new clear fluid appear. Never had any air after the first few strokes. Suspect the piston switch is just stuck. Am going to put caddy up on hoist and loosen brake line going from the cast iron housing for the brake alarm to the front brakes. Then will have my wife gently press brake pedal down a bit while she watches brake light to see if piston moves away from rear brake line position towards center. That might do it. If not, will investigate the switch on the vacuum booster housing as you suggested.

May have to wait until I put a water pump on my 95 beemer tho. Have just found out that it is just beginning to leak while doing an oil change on it. Always something it seems. Last 3 yrs I did not do any work on my 3 vehicles other than oil changes and tire rotations. When it rains it pours.

Gerry :cool2:

10-22-11, 09:55 AM
This happened to me but it turned out to be a stuck piston in the prop valve, the old valve also wasnt letting any fluid to the back brakes. Soon as we put the new one on, we got fluid to the back wheels and noticed a cylinder was leaking, probably the root of the problem to begin with. I was able to turn the brake light off by disconnecting the brake switch at the prop valve though so I dont think your problem is at the valve. I was also able to look into the prop valve and see the piston out of place, I could move it with compressed air. Not sure whats going on with yours but if disconnecting the switch at the prop valve didnt turn off the light, I dont think your problem is air in the system.