Brake light contantly on

[Chris Lovell]

1988 911 Cab
The brake lights are always on now. Over the years, I have replaced the hydraulic brake sensors several times on the 911s but always to solve the brake light never coming on, or intermittent. This is a first. Additionally, the front brakes (both sides) are not fully releasing when I stop. The result is the car will not roll freely very well. This is also unusual. The worlds best 911 local tech suggested bleeding as option #1 and new calipers as option #2.
Do you think the brake light always on due to constant hydraulic pressure (and not a faulty switch) and the almost frozen front brakes are related? And if so, what element in the system needs replacement/repair? Master cylinder?
Lastly, the pedal is hard and the car stops well.
Thanks,
CL

[Joe V]

Binding binders might cause a feedback issue that can contribute to the issue you are having. I have to recommend to rebuild/replace your calipers. Rebuilding is a pretty straightforward operation with seals readily available from our host. Though I split the halves and replaced the seals, I would recommend against it. Just complicates things and increases time and expense. Mine starting pulling hard to the left after a 2 day DE at Sebring (darn that was fun!!!).

Others will chime in on this, but check your brake light bulbs. My '84 had 1157 bulbs, but it should take 7528 bulbs. Very similar but slightly larger base. My brake lights were staying on constantly like yours, and I traced it to a faulty BRAND NEW brake light switch. I put the old one back in, replaced the 1157's with 7528's (which fit more snuggly) and lubricated the sockets with some spray lithium grease to keep the corrosion down. She's been fine for about 2 weeks in the humid Tampa weather.

Oh, and Happy Thanksgiving and drive safely!

[Chris Lovell]

Thanks Joe. And Happy T-day to you as well.
All your thoughts are on target and worth investigation. But I think that the fact that both front wheels have equal turning resistance has me thinking upstream of the calipers. Is there some sort of check valve at the master cylinder which releases hydraulic pressure once off the brake? In most master cylinders, the piston presses past the inflow from the reservoir to pressure the lines. Then, when the pedal is released, the piston returns and the reservoir can reload the lines (as necessary). Maybe the 911 works differently. Maybe I am completely off target.
CL

[911pcars]

Master cylinders in disc brake systems contain no residual pressure valves or other pressure restrictors as found in drum brake systems.

On your MY 911, a brake pressure switch on the MC provides the current path to the brake light circuit. The switch is a common service issue. Check the archives for lots of info on this factory weak point.

"Binding binders might cause a feedback issue....." ??

Sherwood

[Joe V]

911pcars,

Referring to a possible brake hose that's failing internally and causing a pressure buildup. Sorry, should have clarified.

But if both front wheels are dragging, then that might not be the issue at hand.

The most simple solution would be sticking pistons (both front) and a faulty (or pair of faulty) brake light switches.

[ossiblue]

Many years ago, I had a similar problem with a non-P car which turned out to be contamination (oil) in the hydraulic fluid. It caused the exact same symptoms--hard pedal, brakes that wouldn't release, brake lights always on. Cure was a system bleed and caliper rebuild. This may or may not be your issue, but it sounds like some obstruction rather than the switches.

[911pcars]

Originally Posted by Joe V

911pcars,

Referring to a possible brake hose that's failing internally and causing a pressure buildup. Sorry, should have clarified.

But if both front wheels are dragging, then that might not be the issue at hand.

The most simple solution would be sticking pistons (both front) and a faulty (or pair of faulty) brake light switches.

To clarify: If both front brakes are dragging due to blockage in both brake hoses or binding in the caliper for whatever reason, those conditions don't affect the system pressure generated by the master cylinder. The brake pressure switches react whenever pressure is produced in the MC. The calipers are mere slaves of MC pressure - they don't in themselves create system pressure.

Sherwood

[Quicksilver]

The rubber brake hoses at the wheels can swell internally so they retain pressure and that would cause the brakes to drag, but that pressure would be at the individual wheel and the chance of it being equivalent on each side is amazingly small. This also couldn't cause residual pressure at the brake light switches which could keep the brake lights on.

The easy test would be to slightly loosen each brake light switch to see if it releases pressure and allows the brake lights to go off. (Which I suspect you will find will happen.) If there is pressure at the switches then the blockage/failure is upstream in the system.
In other words it is one of the following:
- An odd failure of the seals in the master cylinder that is blocking the port from the reservoir to the cylinder.
- A problem with the brake booster that is keeping the MC piston slightly depressed so pressure is retained.
- A problem with the pedal linkage that is keeping the pedal slightly pressed.
- Or something really odd that I can't imagine.

I would first slightly loosen the switches to see if they release some pressure and turn off. (Watch out for dripping brake fluid as it will eat paint.) If that shows a problem I would then tighten it all back up and use the system until the problem manifests itself again. (Probably instantly...)
Once the problem is occurring again I would loosen the MC from its mount on the brake booster. (Don't loosen anything else.) If pulling the MC slightly away from the booster doesn't make the brake lights go out then the problem is definitely in the MC and it needs to be replaced. If loosening the mountings turns off the brake lights then the problem is in the booster or the pedal linkage.

[Chris Lovell]

Very good ideas Quicksilver. I'll try the switch release and MC loosen. THat will give some better idea.
Thanks,
CL

[OldTee]

Easy to check. Remove heater cover from front trunk. Simply pull the connector on the MC. First one then the other. Replace shorted switch. Problem solved.

[RWebb]

if your rubber brake hoses are over 10 years old, or the age is unknown, then change them out for new rubber even if they are not the likely cause of your current problem

when you remove them, it will be a good time to fix/inspect the rest of the system

change your brake fluid at least every 2 years

[Chris Lovell]

I will do jus that RWebb. The lines are 21 years old. I replaced the line on the '86 with stainless which is hotly debated. But for this street Cab, new rubber is in order and a good step in this repair. Of course new fluid as well. I think I may bleed the old fashion way instead of the Motiv pressure bleeder. Might be better to push whatever obstruction I have out.

[Chris Lovell]

All,
I finally got to the front brakes as an end of winter project. The outside caliper pistons on left and right front are/were frozen. Rebuild kits from our host are on the way. This was Joe V's original idea and was right on apparently. I'll let you know if it works out, which I am sure it will.
Thanks,
CL

[RWebb]

Good Luck!

[Chris Lovell]

Okay.
Calipers are rebuilt and in. After a pressure bleed, they seem to work fine. My brake lights are still constantly on however. The problem is with the pressure switch closest to the driver on the MC. It has continuity when the pedal is not pressed. Interestingly, continuity flickers when the pedal is pressed (just when the pedal is first touched). Is there anything I can do to repair this switch? Do you think a "manual bleed" using the pedal might help?
Any thoughts are appreciated.
CL

[TibetanT]

Chris:

What is the condition of your brake sensors? Could they be in need of replacement and causing this "brake light aways on" condition? Just a thought.

[dshepp806]

Maybe I missed it,..but did you get new switches installed?

Doyle

[Chris Lovell]

I probably wasn't clear.... My rear brake lights are always on with the ignition. This definitely has to do with the pressure switch on the master cylinder closest to the driver/firewall. I have ordered replacements and have yet to receive them (not happy about that). In the meantime, I was hoping one of you have figured out a work around. I am thinking i can wire both connectors into the working front pressure sensor as a temporary solution.

[porschenut]

Chris, have you tried just disconnecting the bad one? The switches are wired in parallel so if one doesn't work the other will still actuate the brake lights. As I understand it, the only thing you would lose by disconnecting one is the function of the brake pressure warning light on the dash. I read a Panorama tech article on this last night.

[Chris Lovell]

Oh for God's Sake! It works fine with just uplugging the front sensor. The pressure (no pun intended) is off now and I will install the new switch when it arrives.
Thanks porschenut for the solution.
CL