Brake Light is on

[84TD]

What sensor would cause the brake light to come on a 1984 F250 brake systems? I thought it was the weeping wheel cylinder so I replaced it, bled all four and its still on. Also the brakes feel very weak, almost like there is something wrong with the assist but the vacumm is in spec.

I did check the e brake sensor too.

 

[79jasper]

Low vacuum will do it. I'm not sure if you have the rear ABS sensor on the rear differential. But if so, it could do it.

 

[madpogue]

In decreasing order of criticality:
* Hydraulic failure
* Vacuum failure
* Parking brake applied

How low did the brake fluid in the master cylinder get with the wheel cylinder leak?

How are they "weak"? Too much pedal effort? Too much pedal travel?

 

[madpogue]

Disregard - double post

 

[Kevin 007]

Mine does it from time to time. I did away with the parking brake switch and nothing was gained. My brakes work well and don't leak, so I guess it could be vacuum. Where is the vacuum sensor anyways?




Im not too concerned about it....yet.

 

[franklin2]

You have a earlier truck without abs, so I am not sure how it's equipped. If you have a proportioning valve under the master cylinder, look for it and if it's like the gas trucks, it will have a wire going to it. Unplug this wire and see if the light goes out. If it does, the proportioning valve has become off-center because at one time or another you had a brake failure in either the rear or the front, or during the bleeding procedure it got off center.

My truck also has a vacuum switch mounted on the pass side fender near the starter solenoid area. It looks like a round thing with wires hooked to one side, and a vacuum line hooked to the other. I am assuming this was used in case you had a vacuum system failure, and that would alert the driver that they would have no vacuum boost for the brakes. Anyway on my truck, this also will bring on the brake light. If you have this thing, unplug the wiring plug and see if the light goes out.

 

['94IDITurbo7.3]

sometimes when i start my truck the red brake light will stay on for a few min. vac is good and no leaks in the system. probly just wiring issue in my case.

 

[MedicMoe]

I had the same problem with mine. But my pedal would slowly go to the floor. I replaced with a new master cylinder from napa @45.00 and new fulid, bled all 4 corners and everything is good as gold!

 

[84TD]

No ABS, way too old

It completly ran out on the rear a few times. I think thats when the light came on but it has been on so long I cant really remember. I was just going to overhaul the entire system when I add hydroboost but the budget just has not allowed it so I need to just get it straight and safe. Is there a way to reset the prop valve? I hope I dont have to replace it, looks like a real pain where it is.

Vacuum is around 20hg

 

['94IDITurbo7.3]

what does teh vac go down to when you pump the pedal 2-3 times?

 

[redneckaggie]

more importantly how fast does it rebound

 

['94IDITurbo7.3]

good point as well ^^^^^

 

[franklin2]

To make sure it is the proportioning valve, I still would try to disconnect the wire to it to verify that is what's making the light come on. If this wire simply rubs through the insulation and touches metal somewhere, that will turn the light on also. The proportioning valve grounds this wire to make it light up.

As far as re-centering the valve, the only thing I can come up with is to open one of the bleeders on one of the front calipers, and then stomp on the brake pedal. This theoretically will push the piston back to the other side in the valve, but I have never tried this before.

 

[84TD]

Bleeding the front brakes recentered the prop valve and the light went off although the brakes still feel spongy. I think since the rear went dry it needs the MC to be bench bled, if I got to do that I am going to replace it since its only 25$.