1972 Dash Brake Light

[kna4977]

Was messing around in the engine bay and the electrical plug that goes on to the switch on the master cylinder didn't seem to be fully on, so I pushed it down to where it would seat. Dash brake light came on and stayed on.

Took the plug off and someone at some point had put tape on it I assume to keep it from staying on all the time. I retaped it and put the plug back on. The dash light comes on when the key is first turned on then goes off like it should. I believe this is how it is supposed to function, but don't think tape would have ever been involved originally. What do I need to look into to get it working like it should when fully connected without tape interrupting the circuit?

As far as stopping goes, the truck does like I think it should.

[vics stuff]

If I am correct , I think the dash brake light that is tied to the proportioning valve on the master cylinder , is telling you that there is a difference in pressure between the front and rear brakes and either the system needs to be blead or there is something going on inside the brake cylinder. I had found this to be the issue with my all stock 72 two wheel drive .
The light should not come on at all even when initially starting your engine . Something is going on

[zosoppp]

Originally Posted by vics stuff

If I am correct , I think the dash brake light that is tied to the proportioning valve on the master cylinder , is telling you that there is a difference in pressure between the front and rear brakes and either the system needs to be blead or there is something going on inside the brake cylinder. I had found this to be the issue with my all stock 72 two wheel drive .
The light should not come on at all even when initially starting your engine . Something is going on

There is a bulb test circuit for the brake warning light and the temp warning light when the key is in START (not run). It should be off at all other times.

[PbFut]

You likely have a out of balance brake system as noted above. Searching will yield decent info on subject. It could also simply be the switch. Removal will not cause leak. Testing is simple. Inline tube has both style replacements. If the suttle valve is tripped causing light swith to trip, you are only stopping with half your brakes and you need to figure out the problem.

[dmjlambert]

I recommend follow the brake warning light test procedure on p 5-23 and 5-24 of the 72 service manual. If you don't have the manual, a PDF version is available here on the forum.

But first if your brake hoses are old, replace them and also check that you can bleed all 4 brakes and fluid flows very well, and if it comes out at the brake cylinders/calipers dirty or different color than what is in the master cylinder, flush out your brake system with new fluid completely.

Back to testing the switch, to reset switch, apply heavy pedal force. This force will apply hydraulic pressure which re-centers the switch contact. The manual says failing the check the entire combination valve must be replaced, but if you have an original, I don't recommend doing that or you will be stuck with one of those brass block style junk from China and you may join the many people complaining here on the forum about leaky or faulty combination valves. Instead send the original off for restoration at White Post Restorations. I did a lot of reading here on the forum before deciding to get an original and send it to them.

[davischevy]

Originally Posted by dmjlambert

I recommend follow the brake warning light test procedure on p 5-23 and 5-24 of the 72 service manual. If you don't have the manual, a PDF version is available here on the forum.

But first if your brake hoses are old, replace them and also check that you can bleed all 4 brakes and fluid flows very well, and if it comes out at the brake cylinders/calipers dirty or different color than what is in the master cylinder, flush out your brake system with new fluid completely.

Back to testing the switch, to reset switch, apply heavy pedal force. This force will apply hydraulic pressure which re-centers the switch contact. The manual says failing the check the entire combination valve must be replaced, but if you have an original, I don't recommend doing that or you will be stuck with one of those brass block style junk from China and you may join the many people complaining here on the forum about leaky or faulty combination valves. Instead send the original off for restoration at White Post Restorations. I did a lot of reading here on the forum before deciding to get an original and send it to them.

What he said.

[kna4977]

Originally Posted by dmjlambert

I recommend follow the brake warning light test procedure on p 5-23 and 5-24 of the 72 service manual. If you don't have the manual, a PDF version is available here on the forum.

But first if your brake hoses are old, replace them and also check that you can bleed all 4 brakes and fluid flows very well, and if it comes out at the brake cylinders/calipers dirty or different color than what is in the master cylinder, flush out your brake system with new fluid completely.

Back to testing the switch, to reset switch, apply heavy pedal force. This force will apply hydraulic pressure which re-centers the switch contact. The manual says failing the check the entire combination valve must be replaced, but if you have an original, I don't recommend doing that or you will be stuck with one of those brass block style junk from China and you may join the many people complaining here on the forum about leaky or faulty combination valves. Instead send the original off for restoration at White Post Restorations. I did a lot of reading here on the forum before deciding to get an original and send it to them.

Section mentions when bleeding the brakes the pin in the end of the combination valve must be held in the open position by installing tool J-23709. I've bled a lot of brakes and never had to do anything like this. Is this completely necessary and is there a substitute for this tool I don't have?

[PbFut]

Right after purchasing my truck I bled the brakes without the tool no problem. Later replaced the front calipers and now seem to need the tool. Unsure the reason. Maybe cleaner system allows shuttle to move easier. You can get a tool on ebay. I tend to think the plastic ones are better as you have less chance of buggering the valve. You can look down the passage for the switch and see the v in the shuttle to verify its in correct position. If you know the switch is good, you can simply verify no continuity to ground if your dash light is in question.

[kna4977]

I have Z53 gauges so I have temp gauge instead of light but I do have the brake light. How exactly is it supposed to work? I assume it is supposed to illuminate with key on even if there is no issue? One post above confirms this while another disputes it.

If there is an imbalance of pressure I imagine that the valve inside that has the V channel for the sensor would move slightly one way or the other and contact the tip of the sensor completing the circuit and cause the light to come on?

If what I think about the light function is correct, it seems like before I messed with this the light was at least functioning like it should. Not sure how but it was even though there was tape on the sensor preventing electrical connection.

I did some testing today. Checked the fluid it was a little low so I topped it off. No signs of leaks at all. Pulled the sensor and did the reset procedure and reinstalled. Now if the sensor is fully connected the light stays on all the time. If I put tape on it no light at all unlike before when it would at least come on with the switch. Attached are a couple of pics of the valve with the sensor removed.

[dmjlambert]

If you think the sensor is in the v-channel and it is not off center, it is possible the sensor is too long and touching the bottom of the channel. if you leave it unscrewed by a couple of threads to turn the light off, and you open a bleeder on the front to create a leak, and press the brake, does the light come on? And closing the bleeder and opening a bleeder in the back and press the brake does the light go off while it is traveling across the V and turn back on to indicate a leak at the back? If so, I suppose you could shim the sensor.

With the sensor disconnected (or operating properly), the key switch in the crank position temporarily grounds that wire to test the brake warning light. I would not be able to explain why that quit working when it worked before.

[dmjlambert]

"pin in the end of the combination valve must be held in the open position" as the manual mentions is also something I have not had to do. I just bleed the brakes. That pin is part of the metering valve, doesn't keep the brakes from getting pressure, just delays the pressure to front calipers a little bit. It is common lore there is necessity to use a centering tool made of plastic or metal screwed in place of the switch to bleed the brakes. I don't believe that is true, and it would hinder the test of the brake warning light function.

[PbFut]

The switch is a simple normally open puch to close switch. If the switch is removed it can be tested by messuring continuity between the body of the switch and lead pin. It should be open normally. Push in the probe side of switch that rests inside the valve. It should close the switch and create a connection between body and lead pin. You don't need to move the probe much. Think how shallow that v is in the shuttle. When the shuttle moves off center, the v pushes out from under the probe of the switch. That small difference in space pushes the probe up and closes the switch. When you understand how the mechanism works it's easier to identify the problem. Tightening down the switch just a little too much can close the switch. Remember, the v in the shuttle is not very deep.

[PbFut]

As for the valve itself. The device we commonly call a proportioning valve is a actually a combination valve. There are 2 valves in the housing we all reference to as a proportion valve. It has a safety feature, the shuttle valve, and a proportioning feature. The safety feature isolates the front or rear curcuit if a excessive amount of fluid is going to it as a result of a leak. It can see a bleed procedure as a leak. If you damage a line or hose, that curcuit is isolated by the shuttle valve. When the valve slides to isolate a curcuit, it trips the electric switch close and the light on dash comes on by finishing the light circuit to ground.
The other part of the combination valve is the proportion function. It is the small cylinder shaped section at the rear of the combination valve assembly that the rear circuit line is attached to. This is the actual proportion valve. It slightly reduces pressure going to the rear circuit relative to the pressure going to the front circuit. There is no delay in this process. The reason for using a proportion valve is mostly do to a difference in required pressure to slow the wheel at a given pressure in the brake line. The front curcuit with the disc brake needs more pressure. There is also brake weight transfer and other dynamics in play to set up a brake system properly. This is why there are 2 different combo valves sold. 4 wheel and 2 wheel disc brakes.