74 duster brake light switch

-
mopar56

mopar56

Well-Known Member
Joined
Jun 19, 2014
Messages
930
Reaction score
341
Location
Vancouver island British Columbia
so the brake lite on the dash came on in my sons Duster the other day, the brakes work fine and fluid is up, the only way to get the lite of is by unplugging the switch on the proportioning valve, the fluid is rather murky looking and I am wondering if this is having an affect on the switch?, I tried to by a new switch but the parts store only had a double pin switch and this one is single, I thought I would start by removing the existing fluid and bleeding the brakes, has anyone tried cleaning one of this switch's? what do they look like on the thread end?, thanks
 
Check to make sure the parking brake lever hasn't been pulled. That will also activate the brake light...
 
Some of the late model proportion valves had a small pin sticking out on end.
I don't remember if that was for centering, resetting, push it or pull it ?
A factory service manual will have those details. Regardless, The valve did move one way or the other to activate the warning lamp. Has to be a either a small fluid leak somewhere or master cylinder itself failing. If I couldn't find a fluid leak with pressure applied, I would replace master cylinder along with renewing all the fluid. Afterwards you may still need to research the proportion valve.
 
its not the parking brake lever switch that was my first thought it has no effect when unplugged the switch on the proportion valve is the culprit, I really don't see the master being at fault either as the brakes work just fine, my thought is that the contaminated brake fluid has jammed the switch, I was wondering if it was like a reverse lamp switch that has a dent ball on it that may be jammed?, I will try to remove it later and see if it can be cleaned
 
mopar56 said:
its not the parking brake lever switch that was my first thought it has no effect when unplugged the switch on the proportion valve is the culprit, I really don't see the master being at fault either as the brakes work just fine, my thought is that the contaminated brake fluid has jammed the switch, I was wondering if it was like a reverse lamp switch that has a dent ball on it that may be jammed?, I will try to remove it later and see if it can be cleaned
Click to expand...

Reverse lamp switch in on the trans. Not relative to the brake issue.
If the master cylinder bypasses internally it will activate that switch. It is a warning devise and it did exactly what it is supposed to do. I aint gonna argue with you though. Good luck.
 
mopar56 said:
t when unplugged the switch on the proportion valve is the culprit,
Click to expand...

You are in the zone of the problem

Either the warning switch is stuck or defective, or you have a problem in the system causing an imbalance which is activating the switch.
 
Redfish, I know where the reverse switch is and that it isn't relevant I was just making reference to how it operated, and I realize the purpose of the switch BUT what I am getting at is it possible the switch can be triggered by something other than a master failure, such as contaminated fluid or simply a failed switch?, if not then obviously I will require a new master, I am just trying to diagnose first before throwing parts at it, thanks
 
mopar56 said:
I am just trying to diagnose first before throwing parts at it, thanks
Click to expand...

I normally agree, but sometimes that gets to be "too much."

There's only so many things................

a gummed up switch, stuck. Very likely

Differential pressure activating the switch, what can cause THAT

You have a good hard pedal? Any leaks? If this part is OK, then it's not a leak per se, or shoe adjustment

Do the brakes actually seem to work well?

That would seem to indicate a stuck switch

If you refer to the service manual, you get into hydraulic pressure checks. This is messy and time consuming, and in this day and age, not only are inverted flare fittings and adapters somewhat expensive, but many parts store people don't even know what they are

You might get two people and try "gentle bleeding" of one side or the other of the switch to "joggle" the piston inside.

FRANKLY if the brakes work well-------go out in a deserted area and really get into them, I'd pull the wire off and would not look back.

I've had 3 brake failures that I can specifically remember, one on my vehicle, two others on friends cars. IN NONE of them was the warning light "any help." I already knew from "what my foot was doing" that "we" have a problem
 
This is another of those things I picked up while playing hookie from school and hanging around a garage back in the early 70s. A customer would come in and say, " I got in a panic stop situation yesterday and hit the brakes HARD ! Now this brake light wont go off ".
The mechanic would look everything over, do something at the proportion valve to correct the fault ( make the lamp go off ) and tell his customer, " If it comes on again in a day or 3 of normal operation, bring it back. We'll likely need to replace the master cylinder ".
It'll be much easier to ignore me and leave it unplugged.
 
ok guys, all points taken and I DO appreciate the help, after I bleed/flush the brakes with fresh dot3 I will let you know what happened, and to answer all questions yes the brakes work well and no leaks or pedal issues
 
Pardon the interruption, but let me throw in my $.02. The block you are describing has a piston in a chamber between the front and rear circuit. As long as there are no leaks the piston stays in the middle of the chamber and the brake light stays off. When a leak develops and the piston gets to the end of its travel, the light comes on. Just below the terminal for the wire in question, there is a brass plug. If I remember right it's 1/2" hex. Remove it and you can reach in with a pocket screwdriver and work the piston back toward the center of its travel. If that don't work the large flat hex plug on the end of the block will give access to take the whole thing apart. Reset the piston and if you haven't found a line or hose leak before the light comes back on, it's the master cylinder. When bleeding DO NOT push the brake pedal to the floor. If you go farther than halfway to the floor the silly thing trips and you have to reset it. Hope this helps.
 
You must log in or register to reply here.
-

Back
Top