Brake lights are stuck on

[38plymouth]

I'm so frustrated right now. I started a little project last month that involved new front wheel cylinders, turned drums, new shocks, powdercoated wheels with new pinstriping and new tubes. I just got it all back together and it looks great and the brakes work beautiful but, my brake lights are stuck on. What could have caused this? I'm assuming somehing to do with bleeding the brakes or the shoe adjustment. I'm not sure what to do next. I wondered why the battery was dead when I tried to start it.

[Don Coatney]

Most likely your system is holding pressure OR the brake light switch is defective. The switch works on pressure in the system. Slowly open a bleeder screw. If fluid comes out without pushing on the pedal and the brake light turns off you will know the switch is not the problem. If you get no fluid and the light does not turn off then replace the switch.

[38plymouth]

I did notice that after the bleeding the brakes they fully engaged when the pedal was about halfway to the floor. After driving the pedal only moved about 1/4 of the way. They don't feel like they are dragging though.

[LAKOTA169]

I had this problem on my '37. Don't know what bulbs you have in your tail/brake lights, but mine are double filament with the location pins on base that are NOT offset. Had the bulb in 180 degrees off. Rotated bulb a half turn and everything was ok. Something a little off topic. I had a burned out tail light bulb, and all I had for spares were the ones with the off set pins (someone had given me some). Won't work? Wrong. I took a needle file and filed the upper pin off and the bulb went in and is still working.

[38plymouth]

It has something to do with the switch, the wires were so hot that I had to disconnect them before they melted.

[_shel_ny]

If you have power disconnected you can use an ohm meter to see if the switch is activated before, and after you attempt to relieve pressure.

 

The switch could be hot anytime it is on for hours at a time. You will still need to find the reason why it was activated.

[T120]

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Edited August 23, 2016 by Ralph D25cpe

[T120]

I did notice that after the bleeding the brakes they fully engaged when the pedal was about halfway to the floor. After driving the pedal only moved about 1/4 of the way. They don't feel like they are dragging though.

Interesting the distance your brake pedal travels to fully engage the brakes becomes less after driving for a while. What type of brake fluid are you using - any chance it may be contaminated?

[greg g]

Assure that your fluid return port in the bottom of the MC is clear of rust debris.  Looking in to the MC through the filler cap you should clearly see two holes in the bottom of the reservoir.  Assure both are clear with a bit of machanic's wire or a straightened paper clip.

 

also do you have sufficient free play at the top of the brake pedal stroke?  should be and inch or so before the rod engages the MC piston.  

[38plymouth]

Assure that your fluid return port in the bottom of the MC is clear of rust debris.  Looking in to the MC through the filler cap you should clearly see two holes in the bottom of the reservoir.  Assure both are clear with a bit of machanic's wire or a straightened paper clip.

 

also do you have sufficient free play at the top of the brake pedal stroke?  should be and inch or so before the rod engages the MC piston.

I suspect that it's the free play, I will try to check both of these things today.

[38plymouth]

My brake pedal is so stiff now it feels like there is no free play. I did not touch the rear brakes but I did have the front drums turned and then only did the minor adjustment at the top. I adjusted them until the shoes touched and then backed off slightly. Is it recommended that I loosen the shoes in the front a little more or make the adjustment on the master cylinder push rod? I have never made an adjustment to the master cylinder pushrod before.

[Plymouthy Adams]

ensure two things with the master, first that the internal piston will return to the full rearward stop of the cylinder when pedal is released, second; ensure that the relieve ports as seen through the fill hole of the reservoir are not plugged   and as a safety note..do not try to clear the hole if the piston is not fully rearward so as not to damage a cup..

[greg g]

The mention of pedal travel lessening with use strongly suggests a lack of proper pressure relief when the brakes are released.  I believe that there is also a residual valve in the MC that may not be working as designed or as mentioned the MC piston is not return to its rest position, all could be related to contamination in the fluid.

[38plymouth]

For what it's worth I used brand new fluid and completely flushed the system. The old fluid was not very dirty. If one of the relief holes were plugged would the brakes be dragging at all? They seem to be free right now.

[casper50]

if the pressure cannot release brakes will drag.

[38plymouth]

My brakes aren't dragging at all. In fact they work really well but the brake lights are stuck on. I removed the plug in the top of the master cylinder and I can't see any holes in there but the way this is down low under the firewall it would be nearly impossible so I fished around in there and I was able to locate one hole towards the rear of the master cylinder. Do you usually have to remove the entire top of the master cylinder to see these holes?

[Don Coatney]

I am not familiar with a 38 master cylinder. Does it look like these?

 

 

 

[38plymouth]

No, more of a rectangle with a flat top that bolts on.

[Don Coatney]

Can you post a picture?

[38plymouth]

I'll try but it won't be until late tonight or tomorrow morning.

[T120]

I hesitate to mention the obvious but if an incompatible fluid is added to a brake system it may cause a problem similar to what you are experiencing...

[Don Coatney]

I hesitate to mention the obvious but if an incompatible fluid is added to a brake system it may cause a problem similar to what you are experiencing...

brake-fluid-compatibility-chart.png

Good observation.

[38plymouth]

I used Prestone dot 3 silicone fluid and flushed the entire system with it. I snuck home and grabbed a picture, let's see if I can remember how to attach it.

[38plymouth]

Pic

[DonaldSmith]

Dot 3 and silicone?  I thought that silicone was Dot 5.  Must be something new.