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MG MGB Technical - 79 MASTER CYL SWITCH ???

Looking to replace my brake master cylinder as it losing fluid even though it is not leaking. I strongly suspect the fluid is being drained through the vacuum booster and burned out the exhaust. I do get some white exhaust smoke upon occasion especially at idle and deceleration (high vacuum times).

I notice something that resembles a switch located at the bottom center of the master cylinder. There is also a disconnected plug under the switch.

My question is: Just what does this switch do?

The car stops fine. I am concerned about the loss of fluid and the disconnected switch.



This is the switch to the brake failure warning light. The wires running to the switch are hot when the ignition switch is on, so it would be a good idea to find out what happened to them.

As to the loss of brake fluid, there are two small, rubber O rings associated with that side of the system. If these O rings deteriorate, you can get air into the system and a small amount of brake fluid will be lost through the switch mechanism. Look at the diagram in your workshop manual to see this better.

Les Bengtson

I could never get that damn switch to seal properly. In the end I removed it and replaced it with a short bolt and copper washer. No more leaks! I just hope I have a good enough feel for the car that if the brakes ever fail I will notice without the warning light!
Simon Jansen

Simon. A couple of points.

First, the switch is not designed to seal. The seals are the two O rings, inside the left hand side of the master cylinder, that I mentioned earlier. If these rings fail, fluid will flow past them and out through the open switch. Plugging the hole where the switch goes makes it a closed circuit system and fluid cannot leak out of the system through the balance section of the master cylinder.

Second, having had these systems fail several times over the years, my experience is that the light comes on as you are going "Oh! Sh*t!" as the pedal goes to the floor. The light comes on when there is a catastrophic failure--one that you recognize as the light comes on.

Again, the wires going to the switch are powered when the ignition switch is on and, if the wires are not restrained in some manner, offer the potential of an electrical short and a possible engine fire.

Les Bengtson

"my experience is that the light comes on as you are going "Oh! Sh*t!" as the pedal goes to the floor"

Exactly so. If it is the rear circuit that has failed you probably wouldn't notice apart from the light. If it is the front then the warning light just distracts you moments before you hit something.
Paul Hunt

Paul. "If it is the rear circuit that has failed you probably wouldn't notice apart from the light."

Have to disagree. I have had two rear circuit failures, both due to the use of rebuilt rear wheel cylinders. In each case, you could feel the failure as it happened--the pedal went further towards the floor and braking efficiency was significantly lessened. I know that, in theory, the front brakes "do the majority of the stopping", but a majority can be 51%. The truth is that if either the front or rear system (on a dual system vehicle) is lost you will have significantly less braking efficiency and you will realize that a system failure is taking place as it is happening.

Les Bengtson

This is getting interesting. So my loss of brake fluid could very well be through the failure switch and not through the vacuum booster, or both??

I did notice the switch seems to be wet. I did fob it off as the "PB Blaster" (penetrating oil)I was spraying on all the brake line connections over the last 4 days.

The other item is the plug to the switch does not want to remain attached to the lip of the switch even though I press the two "claw" clips over the lip.

The wires are green and red/green. It does appear the plug will only go into the switch one way. Am I correct in this assumption?

Mr. Brown in his brown truck and uniform arrived today with my new master cylinder. It does not have the switch. So before I dismantle this car, Should I further investigate the switch or just replace the whole thing?

Also, how do I know if the switch is even working?


79 mgb

Gary. I have never seen fluid loss through the servo unit. I have seen fluid loss through the brake failure switch several times. I would consider loss through the failure system to be the primary suspect.

Remove the switch from the old master cylinder, if possible. Most commonly, it is possible, but not always. This switch is of the "normally on" type which means that, when it is out of the master cylinder it can be tested by simply plugging the switch into its circuit and turning on the engine. The brake warning light should be on. Press the plunger on top of the switch and the warning light should go out. If those two things happen, the switch is working as intended. The plunger is held in the depressed position by a rod inside the bore of the balance/warning mechanism. If one side is out of balance--low pressure, the rod is forced to that side by the higher pressure in the other portion of the system. When the rod is forced to one side, a groove in the rod moves over the switch, allowing the plunger to rise up into the groove, lock the rod in place, and turn on the warning light. This characteristic is why you have to unscrew the switch several turns when bleeding the brakes. Information on that in the workshop manual.

If you are having a problem with air getting into the system and fluid being lost, you need to check out the entire system for leaks. This includes checking all of the joints for tightness, all of the rubber hoses for cracks, and inspecting the hard lines for cracks. The hard lines are under pressure when the brakes are applied and, over a period of years, cracks can develop. A through cleaning, then applying the brakes several times, should allow you to see if wet patches appear.

A section of paper towel, taped below the switch area of the master cylinder, will allow you to determine if fluid is being forced out of the hole for the warning light switch. If so, you have a choice of either rebuilding the existing master cylinder or replacing it with a new or rebuilt master cylinder. On all of my cars, I have replaced the old master cylinders, possibly rebuilt any number of times over the years, with new master cylinders. I, then, am working from a known resource and can keep records of when it is rebuilt and what was necessary during the rebuild. Others may have different systems.

I have, if I can find it in the garage, a sectioned RB master cylinder showing the internals of the warning light mechanism. I will see if I can find it and get a decent digital photograph of it. If so, I will post it this evening.

Les Bengtson

Speaking of master cylinder on booster systems in general[not mgb specific] I have seen enough M/C failures to tell you not to rule it out,if you take the M/C off the booster you can usually see brake fluid on the inner seal, check it out. RIC

Les - you and I might notice, but an awful lot of modern drivers wouldn't. MGBs may have near 50/50 front to rear static weight balance but a typical *brake* balance would be 70/30 or 75/25. Front-wheel drive cars, a very high proportion in the UK at least, will have even more front bias.

Whilst boosted twin-circuit systems appear to only have an indirect mechanical connection between fluid and servo, seal failures resulting in burning of fluid are documented, more so with the remote servo.
Paul Hunt

The idea of brake fluid being drawn from the M/C through the vacuum booster due to a M/C seal weakness is plausible and documented. Would this brake fluid be burned up in the combustion chamber and exhausted out as white smoke?

The warning light switch: Not to be facetious, but it would seem, as per your above comments, the warning light is as useful on a parachute after you jumped from the aircraft and pulled the ripcord.

After owning 56 cars in my life, and driving for 47 years I fully realize this 30 year old car does not leave the garage without all fluid level and light inspections.

The wetness I observed int M/C area is from the penetrating oil I've sprayed overe a 4 day period. I have freed all the threaded connections as well as the two 1/2" nuts fastening the M/C to the booster.

I will continue to check for "fresh" wetness during the week ahead.



Yes on the fluid leaking into the servo. I've seen servos half full.
Yes on the resulting white smoke.
Yes when a diff valve seal fails the fluid runs out of the switch.
The differential valve has the nasty trick when both seals fail of cross connecting the two circuits of the system, so they both fail! V Smart, yes??
If the immediate preceding has not happened, then when the light comes on, the remaining brake circuit should work just as well as always, but the pedal will be low and you will think you have no brakes.
If the good circuit is front, brakes are fine, heart isn't.
If the good circuit is rear, heart not too happy too, especially if the rear brakes are not adjusted right.
The switches are usually DOA anyhow.
The Green wire is hot, key on. Tape it up or find something useful for it to do.
The other wire goes to the dash light, hook it up to your fog lights, or use the pair and a relay to make a "you left the headlamps ON again dummy" warning system

I note that all that nonsense has been dispensed with today - My Mazdas have a hall effect fluid level sensor only.

FR Millmore

This thread was discussed between 17/07/2009 and 20/07/2009

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