Bad brake master cylinder lead to brake switch not working?

[dsapern]

All,

Is it possible for a bad brake master cylinder to not provide enough brake fluid going to hydraulic brake switch to close the circuit? I have replaced the brake switch with two brand new NAPA parts and still nothing. I know the wire going back to the brake light is good because they light up when power is applied. A bad brake master cylinder is the only thing that I could think of. Also, do the brakes need to be bleed when changing out a brake switch. It might have been that my original brake switch did go bad but added air into the system when I changed it out. Please provide your thoughts. Thanks. 

[Los_Control]

How are your brakes?

If you have a good firm pedal and brakes work fine, there would be enough pressure in the MC to activate the switch.

 

One complaint about a pressure switch, if you are just crawling along in traffic and barely using the brakes, the brake lights may not come on because you are not applying enough pressure to activate the switch.

One of the reasons I am going with a electric switch ... when the pedal moves the lights come on .... does not require pressure to activate.

 

If you have power going to the switch, you have enough pressure to operate the brakes .... the switch should activate and send power back to the lights if all is correct.

[dsapern]

5 minutes ago, Los_Control said:

How are your brakes?

If you have a good firm pedal and brakes work fine, there would be enough pressure in the MC to activate the switch.

 

One complaint about a pressure switch, if you are just crawling along in traffic and barely using the brakes, the brake lights may not come on because you are not applying enough pressure to activate the switch.

One of the reasons I am going with a electric switch ... when the pedal moves the lights come on .... does not require pressure to activate.

 

If you have power going to the switch, you have enough pressure to operate the brakes .... the switch should activate and send power back to the lights if all is correct.

To be honest, I don't know if the brakes are great. They work. I'm restoring the truck and I've only driven it 1 or 2 miles. 

 

With the switch out of the master cylinder and someone to press on the brake pedal it hardly produces any brake fluid (in the hole that the brake switch is supposed to be in). I would think that it would squirt brake fluid all over the place but it doesn't. 

[Los_Control]

Might be a good time to rebuild the MC ..... or take it apart and put new seals in it.

12 minutes ago, dsapern said:

I would think that it would squirt brake fluid all over the place but it doesn't. 

that does not sound right at all ... sounds like some dirt or scale blocking the passage.

That also explains why the switch is not working ... not enough pressure to operate it.

 

I just looked at https://www.dcmclassics.com/

I do not see the MC kit. I bought a kit from them a couple years ago and was pretty cheap ... maybe $25 or less. They also just redone their website .... maybe give them a call or email them to see whats up with the kit.

Great bunch of guys there and very helpful.

 

Possible you can just disassemble yours and clean it, then put it back together as is and will be fine.

For the effort, I would prefer to replace the seals while in there myself.

 

When I got mine put back together and bled them, it felt fine.

Then I imitated a panic stop and slammed the brake pedal as hard as hard as I could as if I was trying to avoid a accident. .... Pedal went strait to the floor.

Just been honed one too many times and the new seals would not hold. ..... Old worn seals can do the same thing.

 

Thats when I went to the Toyota MC swap and also am going with a electric switch ..... whenever I get the truck wired  🙄

[dsapern]

1 hour ago, Los_Control said:

Might be a good time to rebuild the MC ..... or take it apart and put new seals in it.

that does not sound right at all ... sounds like some dirt or scale blocking the passage.

That also explains why the switch is not working ... not enough pressure to operate it.

 

I just looked at https://www.dcmclassics.com/

I do not see the MC kit. I bought a kit from them a couple years ago and was pretty cheap ... maybe $25 or less. They also just redone their website .... maybe give them a call or email them to see whats up with the kit.

Great bunch of guys there and very helpful.

 

Possible you can just disassemble yours and clean it, then put it back together as is and will be fine.

For the effort, I would prefer to replace the seals while in there myself.

 

When I got mine put back together and bled them, it felt fine.

Then I imitated a panic stop and slammed the brake pedal as hard as hard as I could as if I was trying to avoid a accident. .... Pedal went strait to the floor.

Just been honed one too many times and the new seals would not hold. ..... Old worn seals can do the same thing.

 

Thats when I went to the Toyota MC swap and also am going with a electric switch ..... whenever I get the truck wired  🙄

That's what I'm leaning towards...a bad master cylinder. I purchased a Raybestos master cylinder in 2016 and I think it has a lifetime warrantee. If not, I will get a hold of Steve at DCM classics. I do have him on speed dial. What a great man and a great company to do business with. 

 

Thanks for the feedback. 

[Sniper]

Rather than guessing, buy a brake pressure kit.  It is a guage that screws inplace of a bleeder.  That will tell you lots.

 

If the fluid isn't squirting out at the T that switch ties into that is a problem, tells me no rear brakes to speak of.  Since the fronts and the rears all get the same pressure from the same source I have to wonder how well the fronts are working?

 

Finally, and this is important, you need to make sure your brakes are properly adjusted, otherwise this very issue can occur.  Pressure will not build till the shoes hit the drums and if there is too much gap you cannot build enough pressure.

[Los_Control]

6 minutes ago, dsapern said:

That's what I'm leaning towards...a bad master cylinder.

IMHO if your brakes are working, you could just split the brake line and install a T and run your switch from there. It just needs pressure.

 

If your brakes work ok .... sounds like you have drove it and not worried about the pedal on them.

 

You bought the MC new in 2016 and has very few miles on it while you work on the truck .... is possible it sat dry or low for a time and got some scaly rust in it that flaked off and is now lying in the bottom of the MC blocking the hole.

Possible you can remove the switch, then blow air into the MC to clear the blockage .... then the switch will work until the crud settles back into the same space again.

This new of a MC I would not bother with new seals, just clean it out.

 

@Sniper I would have to look at my old one, as I remember it the switch bolts directly to the MC while a single 5/16" line comes out of the MC and goes back 12" or so to a T. Then 5/16" line to the front brakes and a 1/4" line goes to the rear.

The switch and the lines are separate.   Why I think is possible the brakes work fine, just the switch passage is clogged.

[Merle Coggins]

You have stated that you know that the brake lights work if you send power back to them. However, you haven't specified that you have checked for voltage supply to the brake light switch. If you have no power to the switch it can't power the brake lights when it gets pressurized by the brake fluid. Try connecting the 2 wires together at the switch. Do the brake lights come on? If they do that will verify a switch issue. If not, then you need to troubleshoot the power supply. 

 

You can also verify if you are getting pressure to the switch simply by loosening the switch in the T-fitting and applying the brakes. You should get brake fluid leaking past the loose threads. Or, remove the switch and hold your finger over the hole in the T-fitting while a helper slowly depresses the brake pedal. You won't be able to hold the fluid back with your finger. This will allow you to flush out the brake line and your finger will act like a check valve, not allowing air back in when the pedal is released. Once you have verified that you have pressure at the switch, and you have voltage at the switch, only then can you condemn the switch. 

 

All that being said I will testify that I have experienced short life on brake light switches from Napa. I was replacing them every month or two when I first got my truck back on the road, some 15+ years ago. My local Napa store kept replacing them under warranty, but I got tired of replacing them. I finally gave up on Napa and purchased a Standard brand switch from Rock Auto. That one's been working fine ever since. I cut open the 2 month old Echlin switch from Napa and found the contacts to be oxidized enough to block current flow. 

[desoto1939]

Also when replacing the brake light switch, mechanical one not electric. I also block up and support the brake pedal arm so that the pedal can not get pushed down and then release brake fluid and then let air enter the brake lines.

 

My brake switch on my 39 desoto screws into a brake line and brake light brass connector that is mounted on the frame in the engine compartment area. 

 

Some of the older master cylinder had the brake switch mounted on the end of the master cylinder. Not sure of where yours is mounted.

 

Like stated above if you touch the two wires that go in to the switch and your brake lights light up then you have a complete circuit. If not, then there is an eclectic power issue.

 

So the Master cylinder could also be low on fluid and not enough fluid is being pushed through the line to push the internal contact to activate the connection to make the brake light come on.

 

Very simple switch when you think about how it works.

 

Rich hartung

 

Rich Hartung

 

 

[dsapern]

3 hours ago, Merle Coggins said:

You have stated that you know that the brake lights work if you send power back to them. However, you haven't specified that you have checked for voltage supply to the brake light switch. If you have no power to the switch it can't power the brake lights when it gets pressurized by the brake fluid. Try connecting the 2 wires together at the switch. Do the brake lights come on? If they do that will verify a switch issue. If not, then you need to troubleshoot the power supply. 

 

You can also verify if you are getting pressure to the switch simply by loosening the switch in the T-fitting and applying the brakes. You should get brake fluid leaking past the loose threads. Or, remove the switch and hold your finger over the hole in the T-fitting while a helper slowly depresses the brake pedal. You won't be able to hold the fluid back with your finger. This will allow you to flush out the brake line and your finger will act like a check valve, not allowing air back in when the pedal is released. Once you have verified that you have pressure at the switch, and you have voltage at the switch, only then can you condemn the switch. 

 

All that being said I will testify that I have experienced short life on brake light switches from Napa. I was replacing them every month or two when I first got my truck back on the road, some 15+ years ago. My local Napa store kept replacing them under warranty, but I got tired of replacing them. I finally gave up on Napa and purchased a Standard brand switch from Rock Auto. That one's been working fine ever since. I cut open the 2 month old Echlin switch from Napa and found the contacts to be oxidized enough to block current flow. 

I do have power and when I touch the two wires together the brake lights both work. Also, when I use a center punch and simulate hydraulic fluid and press the plunger in the brake switch it works then too. I just think that I don't have enough brake fluid pressure at the switch port to press in that plunger to complete the circuit. 

[Los_Control]

For a 49-50 chebby truck .... bolt  it to the floor board .... soon as you move the pedal the lights come on. If adjusted correctly. $15 on RockAuto.

No it is not original, will not work for you if you want a 100 point restoration.

 

 

I consider it a safety upgrade.

If your brakes/switch are working perfectly, probably 10% of the time your brake lights will not come on.

If in a traffic jam and you are riding the clutch and just slowly moving along ... put in the clutch and barely use the brakes to come to a stop ... there will not be enough pressure to activate the brake lights. ..... You would have to press on the brakes and do a nose dive to activate the lights.

 

Now if someone following you are not paying close attention and they rear end you .... it could be your fault for no brake lights.

So many cars today have dash cams that would show in court your lights did not come on.

Sitting at a stop sign and just holding the car motionless with light pressure on the brakes .... no lights again ..... sometimes I might be cruising down the highway and something is going on ahead .... I might tap my brakes a couple times just to warn the guy behind me we are slowing down.

 

Pressure switches work, electrical switches are far superior IMHO.

 

A electric switch will not fix bad brakes ... thats a different  story .... I have a choice as to what switch to use .... no question which one I'm using.

[JBNeal]

6 hours ago, dsapern said:

I do have power and when I touch the two wires together the brake lights both work. Also, when I use a center punch and simulate hydraulic fluid and press the plunger in the brake switch it works then too. I just think that I don't have enough brake fluid pressure at the switch port to press in that plunger to complete the circuit. 

Sounds like you have located where to tap in with the pressure gauge...if switch is good and brakes work then MC has a problem; if switch is good and brakes do not work, then brake lights are the least of your problems.