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MG MGB Technical - 1977 Master Cylinder Rebuild Problem

While working toward getting my new 77 B roadster on the road, I started the master cylinder rebuild today.

I have a couple problems. This is the MC that has the integral electric switch and piston for the warning light if one side of the brake circuit fails. It's the one with the rectangular double reservoir.

The part of the electric warning switch that threads into the MC is snapped off flush with the body of the MC.

I tried unsuccessfully to remove the differential piston (number 16, page 157 in the Haynes manual) with air pressure as suggested in the Haynes manual. I also tried unsuccessfully to pull it out with a wire. I did not remove the broken part of the switch before I tried to remove the piston.

So, the first question is does the threaded portion of the switch lock the differential piston in place so that I must remove the broken stub before I can remove the piston?

The next question is about the port that the switch screws into. Is that plastic switch responsible for keeping the brake fluid in the system at that point? In other words, can I run without it if I don
C R Huff

Charley. Yes, the brake warning light switch, on the underside of the master cylinder, does block some of the inner parts from being removed. Much would depend on how much of the switch remains inside the housing. There is a spring loaded plunger which is held in the down position by a raised ring of metal on the "pressure differential piston assembly", part number 16 of the factory diagram of the master cylinder. If the plunger and spring are still in the broken off switch in your master cylinder, the plunger, most probably, blocked the pressure differential piston assembly from being removed.

The brake failure warning switch ("pressure failure switch", item 2 on the illustration) is a standard thread size (5/16" UNF from memory) and made of plastic. It should be possible to remove the broken switch through a combination of breaking off small pieces. Sometimes, heating a small screw driver and using it to melt/break off pieces works. When the remains are removed and the master cylinder fully stripped, the hole can be cleaned of any remaining plastic by running a bottoming tap into the hole.

As to the last question, does the switch provide any form of sealing, the answer is no. Many of us have had the rubber O rings, item 17 on the factory illustration, leak and the leaking fluid drip down through the switch. Such a problem also allows air to get into the braking system. Thus, it is an excellent idea to rebuild that side of the master cylinder and make sure that the O rings associated with the pressure differential piston are in good condition.

Les
Les Bengtson

Thanks Les.

The way my switch is broken, only the plastic threaded portion is still in the MC. The rest of the switch, including the spring-loaded plunger is just hanging on the wires.

So, it sounds like you are thinking that the part remaining in the MC should not prevent the pressure differential piston (PDP) from being removed. Something is making it difficult to remove, and I do need to replace the o-rings because all of the other rubber was quite rotten.

If I can get the PDP out without removing the threaded portion of the switch, then I would prefer to leave the threaded portion in there. I believe if it is still in there, I can fasten the switch to the MC with stainless aircraft safety wire.

If I do have to remove the threaded portion of the switch, I suppose I could plug the port until I find a deal on another switch.

Charley
C R Huff

Okay, I got the pressure differential piston out. It was just stuck too badly to blow out. I drilled out the broken plastic switch stub, and while it had nothing to do with jamming the piston, it allowed me to pry the piston back and forth with a screwdriver enough to free it. Had I known beforehand that the piston had travel room inward, I might have freed it by pushing it deeper in the bore.

For the time being, I blocked the switch port.

Charley
C R Huff

When I rebuilt mine I left out the switch. I just plugged the hole with a standard bolt (can't remember the size offhand) and used a copper washer under the head as a seal. No leaks so far. I should look at replacing it in the future but I have modified my dash so would need to fit a brake warning light somewhere. Another alternative would be a warning buzzer I guess. I would hope that if it ever did fail I would notice either the fluid leaking from the duff part of the circuit or from the reduced braking performance. If there is a failure is that enough to notice though? I can't remember off the top off my head how it's all hooked up? One pipe at the back to both rear wheels and two at the front, one to each of the front wheels? So if it fails you either get braking at the front or back but not both?

Simon
Simon Jansen

Simon. If either portion of the system fails, you will know it immediately. Significantly reduced braking capability. I have, from memory, had two failures with the dual reservoir master cylinder systems. In both cases, I knew there was a problem with the brakes before the light came on. With the design of the MG master cylinder, the failure warning light switch is a weak point in the system. Better to take it out of the system as you have done.

Les
Les Bengtson

Thanks Les, will leave it how it is. Was wary of touching it anyway after getting it all leak free in the first place!

Simon
Simon Jansen

Simon,

When half of the brakes go out, the pedal goes closer to the floor. So, A lot of cars had a brake warning light that was tied in with pedal movement. If the pedal went down far enough, it tripped the switch to activate the warning light. I think Saabs were that way, or maybe Benz. I might be mistaken, but I think it was common in the early days of dual brakes to have no warning light.

Charley
C R Huff

I like to think I have a pretty good feel for the car so hopefully I will notice something like that if it ever happens!



Simon Jansen

This thread was discussed between 05/09/2008 and 09/09/2008

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