The title about says it all, but I'll put a little more detail here. A few things first: the car's a '71 240z, I've only owned her for about two weeks now, and I've put about 200 miles on her so far (mostly driven in the day time).

Today I noticed that the emergency brake light on the speedometer comes on when I fully press down the brakes. The key word here is fully as it only comes on when the brake pedal can go no further. Now, I've only seen the light when I'm in neutral (whether it's because I'm at a complete stop, or in the middle of downshifting). It might be able to come on when I'm in gear, but I haven't tried this yet since I'd rather not lock up the brakes or otherwise cause unnecessary wear. I checked multiple times to see if my hand brake was partially engaged by pulling it up and putting it back down, which solved nothing. 

Those are the facts, but now I'll give some more uninformed thoughts, and non-cold hard facts. There's a chance this has been going on longer than I know since I mostly drive in the daylight, and only felt comfortable downshifting within the past few days. Meaning, either I never had the opportunity to notice the brake light coming on when it does because the sunlight drowned out the relatively weak emergency brake light, or, fingers crossed, this is something standard amongst the 240z's and is no cause of alarm. Nonetheless, I think I would have noticed this sooner. However, I read in the manual that if the emergency brake light comes one while depressing the brakes, then there is likely something wrong. As mentioned before, the light only seems to come on while in neutral because the brakes, while in gear, have, what seems to me, a bit more resistance, by which I mean that (and these are just random numbers) the brakes depressed 80% in gear slow the car at an equal rate as the car in neutral at 87% depressed. This could be a product of engine breaking, or perhaps the brake pedal in neutral is a bit softer, as it were.

Thanks for your time!

Have you checked the fluid level in your master cylinder?

In addition to what’s been suggested above, my money is is on a loose connection on your handbrake switch or a dodgy switch; where the contact or connection is moving with deceleration.

 

Take off the vinyl cover and disconnect this little puppy, then see if you still have the problem. If it goes away, happy days and easy fix with a new switch or cleaning up the one you have.

 

 

If that isn’t the problem, watch this video and test your brake warning switch; which is often mistaken for a proportioning valve. I had thought of it as a proportioning valve and wondered why it had a wire going to it!

 

 https://youtu.be/WwmYHVBLi2Q

 

The purpose of this is to give you warning of a pressure differential in the system which could be a bad fluid leak on a pierced brake line or simply a dodgy master cyl. It could also just be a dodgy valve inside the warning switch.

 

So again, an easy test is to unplug the wire going to it and if you problem goes away then you know where to look next!

 

Keep us posted - always good to learn new things from another man’s afflictions ;)

 

Ps. This is a great read too ...


https://www.classiczcars.com/forums/topic/59039-1972-z-brake-switch-valve/

If your brake switch is activating when "fully" depressed, then you have an inbalance when the system is under high pressure. 

Basically at rest the mastercylinder is open to the reservoirs and the system is atmospheric. Once you press the pedal the ports to the reservoirs are closed and fluid in the front (closest to pedal) is transfered to the calipers and rear to the drum wheel cylinders.

The two systems (front and rear) are balanced and pressure will build up when the pads and shoes make contact. The harder you press the pedal the more pressure applied. Both systems pass through the switch and then the proportioning valve.

If the systems become unbalanced the shuttle in the switch will activate the switch. AK260 posted a good video explaining how it works.

What could cause this pressure difference?

1. A low level in a reservoir allowing air to enter the system or the brakes were not bled enough to remove all air after repairs. The pedal should feel spongy depending on how much air is jn the system.

2. A leaking seal in the mastercylinder allowing fluid to leak back to the reservoir. The pedal should sink to the floor when you hold pressure on it, but the reservoirs don't loose fluid. Engine running when testjng this.

3. A leaking component in the system: caliper seal, wheel cylinder or brake line. Does it use fluid and are their visable wet spots around the calipers, brake backing plate or lines?

Fair chance it is a master cylinder seal and likely one of the two primary seals.

 

Edited October 17, 20204 yr by EuroDat

It seems like my 280 did that when the booster went bad. Do your RPMs fluctuate any when fully depressed? If they go up your booster may have a vacuum leak. 

11 hours ago, Tweeds said:

The title about says it all, but I'll put a little more detail here. A few things first: the car's a '71 240z, I've only owned her for about two weeks now, and I've put about 200 miles on her so far (mostly driven in the day time).

Here's a section from the FSM about the warning switch.  I'd check fluid levels and bleed the brakes first thing.  Check the pads and other parts also.  You never know what a PO might have done.  My calipers were on the wrong sides when I got my car.

The drawing shows the little nipple that the wire attaches to, that AK260 mentioned.

https://www.classiczcars.com/files/category/11-240z/

Here's a picture of the switch that makes the light go on.

 

@Zed Head @siteunseen @EuroDat @AK260 @Yarb Thank you all for your information, and I'm pleased to say that the problem has been resolved. While the problem is fixed, I'm not sure what caused it in the first place so I'll explain what I did. In neutral, with the brake pressed to the point where the light came on, I pulled the emergency brake all the way up and then put it all the way back down. Why I didn't think to do this before making the post is beyond me. Before, when the light did come on, there was a point at which, while depressing the brakes, there was significantly less resistance thereafter (probably about 80-90% of the way to the floor). Now the brake pedal has resistance all the way, and I'd like to say that the brake pedal doesn't even go down as far, but there's a chance that last part is false and I'm just tricking myself into thinking the pedal's range has changed. What might have caused this problem, and why what I did fixed it is beyond me. Whether this was a small, benign issue, or something systemic of a bigger problem may be worth looking into, but I am not knowledgeable enough to pose anything beyond ignorant conjecture. 

I appreciate your guys' inputs, and I'm sorry if I wasted your time by not experimenting more before seeking advice. Hopefully someone with the same symptoms and a real problem will benefit from your guys' suggestions. Until then, we could all throw in a couple bucks and buy me a Dunce hat! 

You’re most welcome chap, but I’m intrigued to know the root cause.

What I would guess at is that your self adjusters on the rear drums may have been stuck such that the rear shoes were very off the drums and travelling a long way before engaging. The action of the handbrake and the brakes together may have somehow released them? But all of this is conjecture!

Let’s hope that has really fixed it. When you next take off the drums have a look at the self adjuster mechanism and see if it’s smooth or needs a good clean. One of mine had rusted solid and one side would brake before the other.

17 minutes ago, AK260 said:

What I would guess at is that your self adjusters on the rear drums may have been stuck such that the rear shoes were very off the drums and travelling a long way before engaging. The action of the handbrake and the brakes together may have somehow released them? But all of this is conjecture!

Let’s hope that has really fixed it. When you next take off the drums have a look at the self adjuster mechanism and see if it’s smooth or needs a good clean. One of mine had rusted solid and one side would brake before the other.

The self-adjuster was probably involved as noted.  You should make a habit of using the parking brake, because it's what adjusts the back brakes.

I was going to ask about brake pedal travel earlier because that's what happens before the brake warning switch is actuated.  But, after just two weeks of ownership, baby steps are best (no offense).  Your brakes probably still need bleeding, the pedal should be high and hard.

More stuff will happen.  See you soon...

AK260 and Zed head are on the mark.

"Pressure will build up when the pads and shoes make contact. The harder you press the pedal the more pressure applied."

The front and rear system is balanced in volume. If the volumes needed to apply the brake are different the pressures will be different as well. In this case, the shoes are not adjusted correctly and the distance is excessive you will get an inbalance in pressure because you need more volume for the rear system.

This system has a self adjusting mechanism that is activated by the hand brake. When you park the car, pull the hand brake on and release it several times. That should correct the distance or the rear shoes.

Sometimes the adjusters will seize over time. You can check this by watching the levers at the backing plates and comparing them. If one moves more than the other then you have a seized adjuster.

When I rebuilt my brakes and after the run in I noted how many clicks the handbrakes makes. If it needs more than two extra clicks, and it doesn't correct itself by applying the handbrake several times, I know I have an issue with the adjusters.

 

Edited October 18, 20204 yr by EuroDat