StpaulF100 Posted August 30, 2021 Share Posted August 30, 2021 I knew I need to replace my master cylinder as it would fade at a stop. So like most people I’ve had all the parts in my cart at rock auto for a solid week. Today I noticed my rear brake lights were stuck on when the truck was off and the headlights were turned off. In my past this had led me to believe I have a failed master cylinder. So I picked one up today from Napa, bench bled it and slapped it on. While waiting for my bleeding partner and finishing cooking dinner the brake light are still on! So yes I need to bleed the brakes and check for leaks but do o Have a bad headlight switch or brake booster as well as a failed master? Just putting this in writing to try and track my next steps. Any help or ideas are encouraged. Thanks. Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 31, 2021 Share Posted August 31, 2021 The brake lights aren't connected to the hydraulic part of the braking system. Instead they are controlled by a switch on the brake linkage up under the dash. It is base part number 13480 in the drawing below, and it seems that you either have a bad switch or something wrong with the linkage. I'd play with the linkage to ensure that there's not pressure on the switch all the time. And maybe take the switch off and check it out. BUT, it goes on in an odd way so take a pic of how it is on - assuming it hasn't been taken off and put back on incorrectly. Link to comment Share on other sites More sharing options...
rcarlisle Posted August 31, 2021 Share Posted August 31, 2021 I went out one evening to get a photo of the brake light switch mounting for a fellow forum member. Couple hours later, my daughter came in and said my brake lights were on. I had moved the switch some for a better photo and it stuck on. I repositioned it and lights went off. All that to say to check the psition of the switch. Or the switch is failing, but when mine failed, it just had random brake lights. Has nothing to do with how the master cylinder works. Link to comment Share on other sites More sharing options...
StpaulF100 Posted August 31, 2021 Author Share Posted August 31, 2021 Gary and Randy- Thankyou for your replies. Indeed it seems I have several problems. The switch is bad or out of position. As you both say if you manipulate it just right the lights go out. I took it out and cleaned up the contacts and put it back. It seems to function better. I will probably order one regardless. Into the brake troubles. I replaced the master cylinder as I’ve had a soft pedal for some time. The old one had the piston stuck in the bore telling me for sure it’s bad. I was able to bleed the front brakes, but the rear bleeders are froze. I sprayed pb blaster and I’ll give it one more shot before getting new wheel cylinders. My only question now is? Is the brake booster also bad? Causing the master cylinder to go out and causing the brake light switch to be out of sorts. I know the only way to tell is to get the brakes bled completely. I’ll update when I can. Please keep up the tips info and generally awesome ideas. Thanks Mike Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 31, 2021 Share Posted August 31, 2021 Gary and Randy- Thankyou for your replies. Indeed it seems I have several problems. The switch is bad or out of position. As you both say if you manipulate it just right the lights go out. I took it out and cleaned up the contacts and put it back. It seems to function better. I will probably order one regardless. Into the brake troubles. I replaced the master cylinder as I’ve had a soft pedal for some time. The old one had the piston stuck in the bore telling me for sure it’s bad. I was able to bleed the front brakes, but the rear bleeders are froze. I sprayed pb blaster and I’ll give it one more shot before getting new wheel cylinders. My only question now is? Is the brake booster also bad? Causing the master cylinder to go out and causing the brake light switch to be out of sorts. I know the only way to tell is to get the brakes bled completely. I’ll update when I can. Please keep up the tips info and generally awesome ideas. Thanks Mike Usually a leaky master cylinder kills the booster. That's because the leak frequently is into the booster causing all sorts of corrosion. So I wouldn't replace a master w/o at least checking the booster to see if it is wet inside, and more likely replacing it. And it is possible that a bad booster would keep the pedal from returning fully, and that might trigger the switch and keep the lights on. So I'd check that booster out very closely as I fear it may be bad. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted August 31, 2021 Share Posted August 31, 2021 Usually a leaky master cylinder kills the booster. That's because the leak frequently is into the booster causing all sorts of corrosion. So I wouldn't replace a master w/o at least checking the booster to see if it is wet inside, and more likely replacing it. And it is possible that a bad booster would keep the pedal from returning fully, and that might trigger the switch and keep the lights on. So I'd check that booster out very closely as I fear it may be bad. Having the booster not return won't disable the brake light switch. It works solely on pedal pressure, and the contacts have to close before you will get any forward movement of the booster pushrod. Signs of a failed booster are a hard pedal and hissing coming from under the dash where the pushrod passes into the booster. Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 31, 2021 Share Posted August 31, 2021 Having the booster not return won't disable the brake light switch. It works solely on pedal pressure, and the contacts have to close before you will get any forward movement of the booster pushrod. Signs of a failed booster are a hard pedal and hissing coming from under the dash where the pushrod passes into the booster. Good point about the switch, Bill. And, with that, I don't know why it seems there are two problems at the same time. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted August 31, 2021 Share Posted August 31, 2021 Good point about the switch, Bill. And, with that, I don't know why it seems there are two problems at the same time. Well, I understand why a bad seal will cause a master cylinder to fade to the floor. But if it is an internal leak (and not wet on the back when changed) then there's not a problem with brake fluid killing the diaphragm. Brake switches eventually wear out. I changed mine in the parking spot when I picked one up at Advance a few years back. Didn't seem expensive (around $10 IIRC) Link to comment Share on other sites More sharing options...
StpaulF100 Posted September 3, 2021 Author Share Posted September 3, 2021 Made some progress. Life happened and had to wait for parts. Masters back in. Rebuilt boosters. And a new stop light switch. Need to put the new rear wheel cylinders in yet so I can bleed the system. One interesting update. Per Gary’s schematic it appears someone or the dealer updated my brakes from Manual to power. My pedal is the one in the manual photo. I’m assuming that may be part of my problems. As well some previous owner lost part#2474 which appears to be a bushing that may take some slop out of the pedal. Thanks for the help and wish me luck in the wheel cylinders. Everything seems to be stubborn with rust. Mike. Link to comment Share on other sites More sharing options...
Gary Lewis Posted September 4, 2021 Share Posted September 4, 2021 Made some progress. Life happened and had to wait for parts. Masters back in. Rebuilt boosters. And a new stop light switch. Need to put the new rear wheel cylinders in yet so I can bleed the system. One interesting update. Per Gary’s schematic it appears someone or the dealer updated my brakes from Manual to power. My pedal is the one in the manual photo. I’m assuming that may be part of my problems. As well some previous owner lost part#2474 which appears to be a bushing that may take some slop out of the pedal. Thanks for the help and wish me luck in the wheel cylinders. Everything seems to be stubborn with rust. Mike. Good luck, Mike! At least you know what needs to be done. And what parts are needed. Link to comment Share on other sites More sharing options...
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