MGB: brake light check switch is on?

I went on a short 'errand' run today. When I got in the car to return home the dash brake light warning switch stayed on. I stopped and checked the level of brake fluid and all was good. I used the the search and only found reference to the brake switch itself right behind the pedal box. What is this dash switch actually for in a 74 B?
Thanks
Paul

This thread might help.

http://www.mgexperience.net/phorum/read.php?1,1037825,1037825#msg-1037825

Thank you Brian.
Seems some are saying it is not necessary at all.
But why did it come on now, I wonder. I see no sign of leaking under the hood or around the lines.
Paul

If you are losing some brake fluid around the brake warning switch, (not the brake light switch on the pedal box), remove the switch and replace it with a 3/8" fine thread bolt, less than 1/2" long and use a copper washer,

The early-to-mid 70s brake pressure failure warning switch/light is the source of much merriment in some circles because it is almost totally useless and because it's fun to make up fictitious purposes for it ("it's the ejector seat switch"). It is not designed to come on on its own. The manual says, "The lamp in the switch will glow, when the brake pedal is pressed[emphasis mine], if any part of the hydraulic system is inoperative or considerable adjustment of the rear brakes is required." It goes on to describe the procedure to test the lamp and the system with the switch.

So if you're seeing the lamp lit when 1) you are not pressing the switch and 2) you are not pressing the brake pedal then there is simply some sort of short in the lamp.

No experienced driver will be alerted by the brake pressure failure warning lamp before noticing really bad pedal response and/or braking. It is a useless and silly thing added to the MGB in the name of safety but dropped after a few years when everyone realized it wasn't really improving safety.

It makes a good ejector seat switch! ;)

Dont appear to be losing fluid
the light is on without me pushing it
the pedal pressure is fine.

I will do a better job, in the daylight, of looking for leaks-then go get a bolt.
Thanks guys
Paul

um...
just thought about this-in other words, there is a live brake fluid line going into the back of this dash switch?
all this time I thought this was an electrical switch.

[quote=paul74]the light is on without me pushing it[/quote]

If you're seeing the lamp lit when 1) you are not pressing the switch and 2) you are not pressing the brake pedal then there is simply some sort of short in the lamp.


[quote=paul74]I will do a better job, in the daylight, of looking for leaks-then go get a bolt.[/quote]

Don't look for leaks, look for a short in the warning lamp.


[quote=paul74]just thought about this-in other words, there is a live brake fluid line going into the back of this dash switch?
all this time I thought this was an electrical switch.[/quote]

You are correct - the switch is only electrical. It is wired to a brake pressure switch somewhere on a hydraulic brake line (I've never noticed where!). Only wires are going into the back of the switch.

Ok then
the bolt is for that elusive pressure switch somewhere on the brake line.
I do just need to figure out the short.
Thanks
Paul

Have a look at this photo of the '69 GT I had. It uses the same "brake distribution valve" as your 74. The valve is just below the brake/clutch MC's. The actual brake pressure failure switch is the white plastic bit sticking out of the valve with the yellow cap (the yellow cap is not original. I capped off the switch as I didn't use this circuit in the car).

I suspect this "brake failure system" was added when the cars went to the dual-circuit braking system and the factory had to provide a means of warning the driver when either the front or rear circuit failed..........in theory anyway. Trust me, it doesn't work.

Inside that brass valve is a "piston" that separates the two circuits from each other. That piston is designed to move backwards or forwards if one circuit fails and the greater pressure of the working system forces the piston to move towards the failed system. By doing this, the piston (being metal) touches a small metal pin that's on the end of the white plastic fitting you see in the photo, grounds the electrical circuit and thus activates the dash light. The light is mounted on a push-switch only so the driver can test the lamp.

The light won't tell which circuit has failed; just that one of them has. Of course, you'll probably feel the pedal go half-way to the floor before the light ever comes on. This happened to me this past summer with my car.

After I rebuilt the car's brakes, I eliminated that distribution valve altogether and created a true dual-circuit system. As mentioned, there's likely a short in the lamp itself if your braking system is 100%.

Should you discover any fluid leaking out or around the white plastic switch, that means either one or both of the o-rings that seal the piston in the distribution valve has failed. There should be no fluid where the switch's pin sits in the valve. The o-rings can be changed with a kit available from the usual suppliers.

Simon I saw that valve in the thread Brian linked to but did not know where it was.
And I understand your explanation better now too. Thanks. I did not lose any pedal, probably a short, and I'll check the valve for leaks.
You connected the rear brake line to one reservoir and the front brake line to the other reservoir of the M/C ?
Did you run new metal brake line or bypass the valve with rubber line?
Thanks
Paul

reread your posts in the afore mentioned thread. You used a T and standard brake fittings? from Moss?

You are correct, Paul. I ran all new copper/nickel lines in the car and attached the rear line to the rear reservoir and the two front lines to a T-fitting (same as the one used on the rear axle for the rear lines) then to the front reservoir. Photo attached. Fittings used were original if still good and some new ones, available from Moss through my supplier in Vancouver.

By eliminating the distribution valve, it removes any issues with leaks and makes the dash switch redundant. Now it just fills a hole in the panel.

I have always had my brake test light come on when I apply my emergency brake. Try playing around with the emergency brake lever. Or unplug the switch at the bottom of the emergency brake handle.
Ken

Thanks for the post Simon. Something never sat right about simply plugging the switch port and maintaining a dual system if there is a seal failure inside the valve. Your solution is a simple,cheap way to get around the dreaded valve leaks for ever and absolutely maintain a dual system. Good thinking.

Thanks Charlie. Everytime I built a brake system, I told myself to remove that valve. After losing the brakes on the car a while back, now was the time to do it. It also allowed me to switch over from the original steel lines to the copper/nickel lines. Much easier to work with.

I should have done it on the GT as well, especially as there was never an intent to have the "brake failure" switch activated. Since I used a Mark IV dash, there was no provision for this circuit anyway. Hence, the yellow plastic cap in the above photo.

Cheers
Simon

The pic is worth a thousand words.
Turns out I do have a minor leak where the line comes into the valve from the m/c.
The reservoir is hardly down enough to notice-but down none the less.
Time to follow your fix and eliminate that valve.
Thanks again
Paul

Paul,
The valve is activated by a pressure imbalance front-rear. Once activiated due to a fluid leak, it will stay on. This can also happen when bleeding the brakes. The fix is to take a small screwdriver and reposition the shuttle-valve under the center of the H-block.

A seal kit is available to rebuild the H-block. If there is ANY fluid in the center of the H-Block, the seals on the shuttle-valve need to be replaced. The seals consist of a small o-ring on each end of the shuttle.

[quote=mrkenmgb]
I have always had my brake test light come on when I apply my emergency brake. Try playing around with the emergency brake lever. Or unplug the switch at the bottom of the emergency brake handle.
Ken[/quote]

It sounds like you're talking about the later-70s emergency brake warning lamp. That's actually a very useful circuit which helps drivers to notice when they've left the emergency brake on unintentionally. Don't disable that!

That warning lamp is a whole different animal than the early-to-mid-70s top-left-of-the-dash brake pressure failure warning lamp.

[quote=paul74]
The pic is worth a thousand words.
Turns out I do have a minor leak where the line comes into the valve from the m/c.
The reservoir is hardly down enough to notice-but down none the less.
Time to follow your fix and eliminate that valve.
Thanks again
Paul[/quote]

Sounds like your warning ltght works as designed. Once its on , it stays on until you recenter the shuttle .

I think we have a winner of the "First Brake Pressure Failure Warning System to actually illuminate the Dash Light" contest! :thumbsup: Congratulations Paul! Now you get to enjoy your prize of "I need to fix the brakes".

Good detective work. Which reservoir-line-to-valve is leaking? Front or rear? If it's just a fitting, you may get away with just tightening it. Just confirm there's no fluid in the centre of the valve at the plastic switch, as Clay mentioned? If not, looks like your o-rings are still good and as he says, you can centre the piston by removing the plastic switch and slide the piston back. It means standing on your head but with a mirror and good light you should be able to see into the valve.

I've got my distribution valve on the bench; I can post a photo with the switch removed so you know what to look for. Stand by.

Here you go. This is much easier to see with the valve on the bench but hopefully it'll help.

Why does everyone worry about this thing? If your brakes fail, you'll know it. If you're not losing fluid and the car stops straight, then don't worry about it.

The rear reservoir was a tad lower than the front.
But honestly I cant tell where the leak is really as the area is not clean enough to see that. I have to clean my engine bay.
There are of course no tell tale drips to follow.

Before I saw the last few posts by Clay and Simon I unplugged the valve, wiggled it a bit, cleaned out some corrosion from the two tiny pins and plugged it back in. I might have even tapped with a screw driver lol but I will only admit that here cause I did not what else to try. I thought if I backed out the top plug I would have brake fluid all over the engine. Now that I have seen your posts I guess I got lucky and by messing with it must have recentered the plunger-so it must have not been off by much.

I put the car up to check the lines underneath and found no sign of leaks there. The wheels are clean. Lowered the car. When I got back in I tried putting pressure on the brake pedal. It very slowly lowered to the floor but only for 1/2" or so. I think that means there is fluid leaking past the seals?

I am going to retighten the nuts around the m/c and valve and keep checking for a week. And prepare for brake work.
Thanks guys
Paul

Your master cylinder is indeed bypassing the internal seals. No fluid on the outside is confirmation of this. Time for a rebuild or a replacement unit. RAY