Constant Brakelight

[longhorn70gmc]

Hello all,
Brake light came on in the 72 Super and I can’t get it to shut off. I looked over the Master Cylinder and the fluid is good. Brake function is great. I checked the E-Brake and it seems good. I am thinking the next step is to remove all the wheels and look over the brake system. I do not see any leaking from looking underneeth, but that doesn’t mean there isn’t one. Anyone have any ideals or input to help me figure this one out?
R/
Billy

[original-71]

iam curious too mine started doing the same a couple weeks ago

[68gmsee]

Did you try to reset the pin on the proportioning valve?

Look through this post:
http://67-72chevytrucks.com/vboard/s...ke+light+stays

[SS Tim]

Billy,
As much as I hate doing this without a schematic in front of me here goes. The brake light circut, I believe only has two inputs. The balance switch and the E-brake apply. If everything seems ok, I'd check a schematic and verify that. Then if, as I suspect, both are make ground switches pull the plugs at the switches to verify that the circut itself is not grounded somewhere. In fact you could also put a tester on the switches as a cross check.
Call me later.
Tim

[leddzepp]

If the prop valve pin needs to be reset, I have heard that slamming on the brake pedal really hard will usually do the trick.

[SS Tim]

In links from a boundstaffpress posting are two great schematics. Thanks for posting these boundstaffpress!
Original thread here
http://67-72chevytrucks.com/vboard/s...d.php?t=185856

Schematics in several sizes

1967-72 Chevy truck Cab and chassis wiring | Flickr - Photo Sharing!

1967-72 Chevy truck V8 and cab wiring | Flickr - Photo Sharing!

[406 Q-ship]

Originally Posted by leddzepp

If the prop valve pin needs to be reset, I have heard that slamming on the brake pedal really hard will usually do the trick.

yeap that will do it

[cdowns]

if you r talking about the brake light on the dashboard the only thing it is is from your prop valve unbalance in the system// any recent work done? have you checked your pads and shoes for excessive wear or fluid leak in the drums


there is no emergency brake light of any type on these trucks

[original-71]

Originally Posted by cdowns

if you r talking about the brake light on the dashboard the only thing it is is from your prop valve unbalance in the system// any recent work done? have you checked your pads and shoes for excessive wear or fluid leak in the drums


there is no emergency brake light of any type on these trucks

yes brake light on dash, put new pads and rotors up front , new pads in the rear no leaks all seem to stop just fine but light still there

[cdowns]

are rear shoes adjusted out to a light drag on the drums? if not can cause the unbalance light on

the slammin the brakes real hard can sometimes reset it for you

[SS Tim]

Originally Posted by cdowns

if you r talking about the brake light on the dashboard the only thing it is is from your prop valve unbalance in the system// any recent work done? have you checked your pads and shoes for excessive wear or fluid leak in the drums


there is no emergency brake light of any type on these trucks

Well if the wire from the cluster connector to the balance switch grounded anywhere along the wire run (not an unlikely event in a 40 year old truck) the brake warning light will come on. If the vehicle has been sitting alot, like this one has, then it is even more likely. If brake work has been done and the light comes on after the work, then yes a balance issue is more likely. A quick electrical confirmation is easier than pulling stuff apart to guess. A test light between the pressure switch terminal (not harness connector) and B+ is all it takes. As to the E brake having a light, for some reason I thought my '72 did and the way the question was worded it reinforced that thought. If anyone wants to add that function it is a simple switch and a bit of wire tied into the existing pressure switch circut.

[longhorn70gmc]

Hello Tim,
I removed the front driver wheel to look over the breaks. This truck has disc in the front and the pads are brand new looking from the prior owner. I just removed the power steering box and hope to have the new one in by the weekend.
I will print out all of the info from everyone and go back through all of the system with tires off and re check for leaks and anything out of the ordinary. The breaks do work very well. I will recheck the master cylinder again too.

I will check back in with you or give you a call when I inspect this thing over real good.
Thanks to you and all for chimming in and offering me some input on this. It is truely nice being able to talk to people whom share the hobby and understand. It makes a great difference. More to come!!!

R/
Billy

[PAWS 72]

I just put in a new circuit board a couple weeks ago and changed all the bulbs and when I plugged it up mine came on for the fist time in humpteen years .Any suggustions.

I did put new pads and shoes on, new rotors on front and turned rears.Had the wife to hold the brake pedal for me,she said it went to floor like it was supposed to while bleeding.good pedal also.no pulling either direction

[PAWS 72]

Originally Posted by SS Tim

Well if the wire from the cluster connector to the balance switch grounded anywhere along the wire run (not an unlikely event in a 40 year old truck) the brake warning light will come on. If the vehicle has been sitting alot, like this one has, then it is even more likely. If brake work has been done and the light comes on after the work, then yes a balance issue is more likely. A quick electrical confirmation is easier than pulling stuff apart to guess. A test light between the pressure switch terminal (not harness connector) and B+ is all it takes. As to the E brake having a light, for some reason I thought my '72 did and the way the question was worded it reinforced that thought. If anyone wants to add that function it is a simple switch and a bit of wire tied into the existing pressure switch circut.

What if the lite comes on. Does that mean you need to take the switch out and move the plunger back and then put the switch back in or replace the switch?

[longhorn70gmc]

Hello all,
I finally finished up the rebuilt steering box and installed it. I took the truck out for a test run and tried the hitting the breaks hard to reset the system and bamm, it reset, no more light. This was a great learning experience and a awesome oppertunity to check the whole system for leaks, fluid levels and break ware (found out I have all new pads). The brake pedal and switch looked good too. I am happy I started this thread, because alot of people needed to ask some of the same questions and it has turned out to be a great oppertunity to discuss and talk breaks amonst ourselves.
I am very appreciatative and thank everyone whom chimed in on their experiences, Thanks to all here!!!!!
Please feel free to continue to chime in and keep asking those break questions. This might even be another great FAQ addition thread for all.
As for my truck, all she needs is new tires and will be a great daily driver.

Please keep the thread going for all the break questions and answers!!!
R/
Billy

[va72longhorn]

As an electrician, I would start with the obvious to narrow it down. If, visually, the fluid and brake system look good and is operating correctly, try unplugging the brake warning wire right at the master cylinder. Someone can correct me if Im wrong, but memory tells me its a tan wire. It should be a "make ground", so the light should go out if the there is a trouble in the brake system. If it stays on, theres a short "to ground" in the wiring harness, unless a emer brake switch has been added. I agree with CDOWNS that it would have been a PO add on. If that is the case, look under the dash around the emer brake pedal assembly and there should be a small "limit switch" attached to trigger when you push in on the pedal. It could just be out of adjustment.

If the dash light went out went out after unplugging the wire at the master cylinder, there's a good chance that the brake warning switch is bad and giving you a constant ground, much like when a temp sending unit goes bad. Again, this is after youre fairly sure that the brake system, itself, is in good working order.

[Sport/Truck]

I had a problem when bleeding my brakes and upset the balance valve. Just unscrew the wire terminal (fluid will not come out due to O rings). Then I used a scribe and moved the plunger back into place.
They make a tool that screws in place of the sensor and hold the valve from moving during brake bleeding.

One of these works great. http://www.performanceonline.com/Com...ve-Bleed-Tool/

[jocko]

if the light is on and the system is functioning properly, does that mean that you're primarily stopping on the front or rear brakes only? Or does it mean that you've lost your duplicate circuit protection because the systems aren't isolated anymore? I've scratched my head a lot about this goofy switch. Many think it is a combo valve, prop valve, etc - I always understood it to simply be a brake pressure differential valve on these trucks. Am I missing something? Good thread, by the way.

Billy, since we're both in ridgecrest, I can't be more than 5 minutes away - let me know if you'd like any help. I can scratch my head with the best of them...

[Ryan5O]

How about that little "button" that you can adjust thats near the brake pedal under the dash. If its not making contact, the brake light will stay on. Then again, that would effect the tail lights. After reading the post again it sounds like your talking about the dash light.

[jjzepplin]

I have this problem on my truck and am wondering if your light ever came back on. Mine has been on for a while and slamming the pedal has not worked. I have just cleaned up all the wiring and have no shorts. When I pull it from the valve it goes out. I am guessing I could do a continuity test on the switch and it will tell me it is the culprit?

[SS Tim]

If you are sure the system is working properly and that it is fully bled then yes diag. it down to the switch. It sounds like you have it figured out though. The switch is turning the light on. Why is the question. I think the switch is replaceable, need to look at a 71-72 manual. The combination valve may be the underlying problem if it is crudded up and frozen. Inline Tube has a lot of brake parts and information on their site.

[longhorn70gmc]

Originally Posted by jjzepplin

I have this problem on my truck and am wondering if your light ever came back on. Mine has been on for a while and slamming the pedal has not worked. I have just cleaned up all the wiring and have no shorts. When I pull it from the valve it goes out. I am guessing I could do a continuity test on the switch and it will tell me it is the culprit?

Hello,
so far, so good on the light. It is starting to get cold again, so I will keep an eye on it. The truck has been getting a good gone through here lately. The brakes always have worked well, but when that light came on I was like what the dog gone has happened. This is a good thread, I am glad to see it still being referenced. Thanks SSTIM for chiming in for our board brother.
R/
Billy

[jjzepplin]

OK so I went after this light with a vengeance. I found two things. I took the valve all the way apart and the shaft that turns on the switch was a bit stuck. However it was not the problem. It was right where it should be, just stuck. I check everything and found the switch itself was screwed too for in setting off the sensor. Backed it out a bit and the circuit was broken. I confirmed this by manually moving the shaft to make the circuit close. Put it all back together and no light. Got dark, ran out of fluid after breaking one of the lines ( would have broke while driving so no bigge ) reflared the line and reattached and ...rain! Needed to wash away fluid anyway. Check back tomorrow with more fluid. Woohoo!

[longhorn70gmc]

Glad you found out what it was, good deal. This will be a great FAQ thread for this issue when it pops up for others.
R/
Billy

[jjzepplin]

Checked the operation of the valve last night and the whole thing seems to brake better and all the wheels seem to be more equal. Could be my imagination though. Will post back when I take it off the stands and go for a real ride. I always test them on stands before I go out and hurt someone.