Brake light sometimes stays on

[kevin_tbird]

The brake light on my 60 tbird sometimes wants to stay on. The entire system is new/rebuilt except for new shoes. If I relieve a little pressure at the master cylinder outlet the lights go off.

Could nearly expired shoes cause this to happen? What else should I check? All four wheels are properly bled with new lines, wheel cylinders and a rebuilt master cylinder and rebuilt brake booster. The car stops fine, although I wouldn't mind a bit sharper stop (new shoes I think).

thanks,
Kevin

[KULTULZ]

Complete hydraulic system overhaul and not hang new shoes?

I would think that possibly you have a brake shoe(s) hanging on a backing plate or weak return springs possibly not allowing all pressure off whatever wheel brake end that is not fully retracting.

When the light hangs, slightly open the brake cylinder bleeders one at a time in a pattern and notice if the light goes off. That will be the offending end.

Otherwise there is something possibly wrong in the MC or the adjusting rod that will not allow full pressure return.

The MC reservoir is not overfilled is it?

[simplyconnected]

Another member had a very similar thread. He had a new (dry) M/C, which makes a big difference. So, I will quote myself:

I think Eric pretty much covered all the bases for just about anything that could possibly go wrong. (Good job, Eric.) He also speaks from experience AND the Shop Manual. It doesn't get any better than that.

I hope you check out these suggestions before buying any parts. This could save you some money whether you or your mechanic does the work. You cite three distinct problems:

The key to this is your brake light switch. It operates with very little pressure. That tells me your brakes are always applied, that the M/C spool never returns, and your brake system never 'resets' itself.

First of all, did you bench bleed your new M/C?

There are two distinctly different adjustments; one between the booster and the M/C, and your pedal bolt (which has an offset adjusting shoulder).

Let's start at the pedal. Can you lift it? Is there any 'jiggle' room (or is it solid)? I'll wait for your answer on this.

Remember, every M/C is slightly different. If you unbolt your M/C and move it aside without cracking brake lines, see if your brake lights go off. Look in the back of your M/C and notice if the spool returns all the way back to the snap ring. <--This is important.

At rest, the spool should return completely and on it's own, which opens the reservoir port to the brake lines. This resets the system, allowing the wheel cylinders to return, and it sets up for the next brake cycle. If the M/C piston cannot come back to the snap ring by itself, the spool is internally stuck which keeps pressure on the brake lines.

Another cause for the M/C to not return is if the booster output shaft is extended too far. Your booster and brake pedal should retract on their own without the M/C. If they don't, find out if the spring inside the booster (that returns the brake pedal) is broken.

The booster should have an output screw with a jamb nut arrangement. Stick a piece of clay (PlayDough, plumber's putty) between the M/C and booster, and bolt them together. Then unbolt them and look at the clay. If the clay is squashed out, the jack screw must be backed off just enough to let it jiggle. - Dave

[Guest]

I bought a bad m/c. my lights would come on in the middle of the night and run the bat. right down. Went and got another m/c and its allright now. ( if it sits awhile i still unplug the wires at the switch)

[kevin_tbird]

The master cylinder was bench bled and the brake pedal has no play in it. The Booster output shaft was set to spec by making a gauge according to factory specifications.

BTW. The old shoes were used because the brakes had been done on the vehicle the last time it was driven ( about 40 years ago). I'm replacing the shoes just in case there is some warping of the shoes.

Kevin

[KULTULZ]

The master cylinder was bench bled and the brake pedal has no play in it. The B Poster output shaft was set to spec by making a gauge according to factory specifications.

If you pull the brake pedal to the utmost top of it's travel, will the brake lamps stay on?

BTW. The old shoes were used because the brakes had been done on the vehicle the last time it was driven
(about 40 years ago).

I'm replacing the shoes just in case there is some warping of the shoes.

Kevin

Please buy a premium quality shoe, replace the hardware (kits) and make sure the backing plates are true.

Complete wheel bearing(s) service, grease seals and the drums are properly machined. This era braking system (BIRD) was undersized on the front due to steer turning restrictions and can lead to severe seat cover sucking events.

[kevin_tbird]

No. When the brake light sticks on, pulling up on the pedal has no effect.

[simplyconnected]

That's because the M/C cannot return.

...The key to this is your brake light switch. It operates with very little pressure. That tells me that your brakes are always applied, that the M/C spool never returns, and your brake system never 'resets' itself...

Try this, next time your brake light stays on, loosen the M/C from the booster and see if your brake lights go off. - Dave

[KULTULZ]

The master cylinder was bench bled and the brake pedal has no play in it.

The Booster output shaft was set to spec by making a gauge according to factory specifications.

That's because the M/C cannot return.

Try this, next time your brake light stays on, loosen the M/C from the booster and see if your brake lights go off.

- Dave

Correct. The pedal asm has to have some play or it is not returning fully.

The gauge shown in the manual is for a completely OEM setup. Any deviation from OEM part(s) will compromise the setting. Use a depth gauge to ascertain proper air gap.

[kevin_tbird]

Note: this is a completely stock POWER brake setup, everything is factory. The brake light sticks on. I am not getting much power assist either.

The brake light is stuck on again. I measure the brake pedal play with the engine off and have about 1/4" of play. Moving the brake pedal does not turn off the brake lights.

I removed the master cylinder from the brake booster. The brake light stays on. I removed the piston between the master cylinder and the brake booster. Brake light stays on.

This is a rebuilt master cylinder that was bench bled. The piston in the master cylinder is returned all the way to the snap ring.

Advice on next steps is appreciated. [ I went back into the garage and moved the MC around while looking the setup over again. plugged back in the brake switch and the brake lights are now OFF]

Kevin

[NYsquarebird58]

Maybe you have a faulty brake light switch? I'd give that a try. I bought a new one from Rock Auto for $12.00

[juantejeda]

Brake Switch

You can get a new Brake Switch at Napa for about 8 bucks.

[kevin_tbird]

This is a new switch.

[simplyconnected]

Kev, when I troubleshoot I normally find the root cause first, then I consider the circumstances (only because I don't want the same thing to happen again).

In your case, everything is new. New components can be maladjusted, the parts haven't been 'proved' to work, and we all realize that some new parts can be defective and new parts usually fail early rather than later.

We have seen a few reasons for pressure switch failure. One cause is from switching from DOT-3 to DOT-5 brake fluid (the seals bloat and it sticks). This switch is meant to switch electrically at a very low brake pressure even though the brake line may reach 2,000-psi.

Without actually seeing your setup, and based on what you describe, I tend to agree with Marcelo. When you loosened the M/C, noticed the spool was fully reset, and the brake lights remained on, leaves only one cause.. a faulty switch.

I happen to have a 'used but still good' spare if you want to try it. Or, take Juan's suggestion and buy another at NAPA. - Dave

[kevin_tbird]

I've gone through the entire system again checking each linkage. The piston rod between the booster and MC was at the high end of the range. I moved it to the low end. Small adjustment to the brake pedal eccentric to 1/2" of play. New shows on the front and carefully set star wheel adjusters. (old brake pads were only slightly worn. Light is off. It is raining right now so no chance for a road test yet. The switch problem was intermittent. I'll post results later.