Brake Failure Warning Switch

[jcatnite]

During the Brakefest '07 on the 76 Midget I ruptured the rear brake hose. After that, I found it extremely difficult to bleed the rear brakes with my vacuum bleeder. In fact I could only get a couple of mils of fluid from each cylinder at a time. I am suspecting that the brake failure warning switch shifted on me and didn't reseat. I have seen on the forum a few times a discussion of how to reset this but I can't for the life of me remember what we called it here. If someone can help me out with the acronym, I don't mind doing the research but without it I can't seem to narrow the search parameters enough to find what I'm looking for.
Thanks,
JC

 

[Morris]

You should be able to pull the threaded switch and pump the brakes to get it to reset. I would just leave the thing off until you have the brakes working the way you want. Otherwise it will just jam up again while you are bleeding.

 

[jcatnite]

Thanks bud, I thought there was some kind of proportioning slug in there but now that I think about it, all the proportioning happens at the master. So, I don't need to plug the port when I take the switch out? I've got a spare one, maybe I'll rip into it first to see what it's made of before I break open the system again.
Thanks again,
JC

 

[Morris]

If you pull the switch and you see oil, you need to rebuild. Rebuilding should be just replacing two o-rings.

 

[jcatnite]

Thanks again. I looked in my manual just now when I got home and it is a pressure differential warning actuator. I think it got tossed around as a pdwa most of the time. There is a procedure for "centralising" it in the manual but it seems a bit hinky. Think I'll try your suggestion after supper.
JC

 

[Morris]

Yours may be different than mine, as I have a later model. Indeed, mine is different than the diagram in the Haynes manual. But...

I was having problems getting my switch to work properly. It seemed that the end of the switch was not even contacting the shuttle-ma-bob in the manifol-dingy. I stuck an extra detent ball I had from my gear box rebuild in the cavity the switch screws into (it were a perfect fit), then filed the switch plunger to fit. Now if the switch is activated, it should just pop back into place once pressure is released.

 

[jcatnite]

Good thinking. Here is a photo of the inside of mine. I didn't really see a valley in the shuttle. What I saw was a thin raised flange that seemed to be right in the spot where the switch plunger should be. Of course the switch isn't working as it seemed to read 1.9Mohm no matter what the position of the plunger might be. I think mine is stuck in a weird spot but I want to get the spare one apart first to see what I'm getting myself into. I couldn't shift the spool with a small screwdriver but I have to admit I didn't fiddle with it too much. Right now I have brakes so at least it's drivable. I cropped the picture up quite a bit but thought you could at least make some sense of the important parts.
JC

 

[Morris]

The switch is odd. I think the way that is works is it grounds itself on the shuttle to turn the dash light out. If the shuttle moves, the plunger descends and breaks the connection with the leads. It also prevents the shuttle from returning to center position. On my switch, the two leads are connected in the switch. I guess the dash light is controlled by a relay. Doesn't seem like you would need two leads for that.

 

[Bugeye58]

Just a simple switch, Morris. When the shuttle valve moves off center, the switch contacts close, completing the circuit to the warning lamp. No relay involved. If you put something in there to impede the movement of the shuttle, a leak will more than likely allow the affected brake system to drain, without any warning light to indicate a problem. That could be <u>very </u>interesting, should you be going into a corner hot, and find yourself with only rear brakes.
Jeff

 

[jcatnite]

My shuttle is in the center. The flange I was seeing is the center of the shuttle. I got the spare one apart this morning. The light doesn't work but at least I know the shuttle is working. I see that I can get a switch portion from the usual suspects so all should be good. So here is a new question. Shouldn't I be able to bleed the rear brakes using a vacuum rig? I had no problem with the fronts but couldn't get anything but a couple of milliliters out of the rear wheel cylinders.
JC

 

[DrEntropy]

Check the flex hose at the rear! It'll likely be swollen internally and block the flow to the rear brakes.

 

[Morris]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Just a simple switch, Morris. When the shuttle valve moves off center, the switch contacts close, completing the circuit to the warning lamp. No relay involved. If you put something in there to impede the movement of the shuttle, a leak will more than likely allow the affected brake system to drain, without any warning light to indicate a problem. That could be very interesting, should you be going into a corner hot, and find yourself with only rear brakes.[/QUOTE]



Well, I think the way the system is designed, the shuttle is impeded whenever it is activated. That is why it is necessary to re-center. With my mod, the shuttle can move off center then return once pressure is released. Of course, I can see how that would cause the situation you describe if the purpose of the shuttle is to "stop up" the bad brake circuit.

And as for my "switch." It is absolutely not a simple switch as you describe. I am not sure why. Maybe DPO put something non-standard in there. But on my "switch," the two leads are crimp connected to a single terminal. It is clearly the way the "switch" was made and not a DPO mod. Unless DPO was an elf with very tiny hands.

 

[jcatnite]

Thanks Doc, I replaced the rear hose already. I am getting some pressure to the rear brakes cause I can hear them operating but I couldn't figure out why I'm not getting flow out of the cylinders with the vacuum rig. BTW, when did you change your avatar? I almost didn't recognize you...lol
JC

 

[jlaird]

Ahhh, something is causing a problem, bad flex hose or maybe even one of those switch things in the line. But yep vacuum rig should work just fine.

 

[DrEntropy]

Thanks Doc, I replaced the rear hose already. I am getting some pressure to the rear brakes cause I can hear them operating but I couldn't figure out why I'm not getting flow out of the cylinders with the vacuum rig. BTW, when did you change your avatar? I almost didn't recognize you...lol
JC

Ah. With a new flex-line the switch would then be suspect, but if it's centered there'd almost have to be something pinched/crimped/blocked in the rearward supply circuit... really odd. You have any way to pump/release in the "traditional way"? I'm curious now as to what ya find!

Avatar: changed a few days ago, may do again. Playin' about with it.

EDIT: Just out of nosiness: Did you check the new flex line to be sure it was clear before the install?

... think "mud daubers" here... and don't ask what would cause me to suggest this. /bcforum/images/%%GRAEMLIN_URL%%/mad.gif

 

[jcatnite]

Doc,
I got the hose from a sealed baggie at the local autoparts store but I have to admit, I didn't check the internals. That has concerned me a bit as it was my first suspect as well. Also the fluid that is coming out of the rear bleeders is dark as cofee. I'm thinking the line might be plugged...sigh...I'm going to try the pump and squirt method maybe tonight. Hopefully, that will be enough to clear the lines and get some fresh fluid in there. Now that I know how to check the PDWA and make sure that it is centered. I'm not afraid to get a little more frisky with it. I was always concerned about shifting that sucker and not being able to get it back to the center. I might jump it out just to make sure that it isn't the culprit.
JC

 

[DrEntropy]

No sense in NOT being thorough with this. It is, after all, the BRAKES. You can limp home on three cylinders, but you'd still be better off havin' All corners with working brakes ALL the time.

Good luck and report back! /bcforum/images/%%GRAEMLIN_URL%%/wink.gif