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MG MGB Technical - Brake failure switch

Restoring a '72 bgt and am working on the brake failure switch unit. I've removed the shuttle valve piston and was going to install new O-rings. But after reading the archives it seems like this is just another potential hydraulic leak waiting to happen and the failure switch doesn't do much good anyhow. Would it be ok to leave the piston rod (shuttle valve) out of the unit and use a 3/8 screw in place of the plastic threaded switch?

Thanks for any advice,
Tom Custer

Dirk Diggler

Your idea of leaving out the shuttle piston
will essentially convert the (stock) dual hydraulic
brake system into a single hydraulic system.

If a leak should occur anywhere in the system,
then, potentially, you can lose all braking.
Daniel Wong

I ran a car for several years with a cap screw in place of the plastic switch. No problems were experienced. I didn't remove the shuttle. As Danny said you will lose the dual braking system feature with the shuttle out. You can reseal the shuttle and it will probably last another 10 years, maybe longer if you keep the brake fluid changed. The shuttle does not shut off a leaking front or rear system, it only turns on the dash warning light and any leaks will continue until the appropriate reservoir section is empty.

Clifton Gordon

Thanks for the info gents. Daniel, perhaps you could guide me on which O-rings to use. I have the Moss 181-995 kit with the two different sizes of O-rings (small and a tad smaller). The piston looks like the one in the middle of the three types shown as "piston types - new design" on the instruction sheet and has a very small dimple in the center of one end of the piston. The instructions say to use the smaller of the O-rings for this piston. However, the original O-rings that I removed appear to be the same size as the "larger" O-rings in the kit. I'm also a bit confused by the statement in the instructions: "The original pistons with flat ends are used with the larger pair of O ring seals." Don't all of the pistons have flat ends?

Muchas gracias for your help!
Tom Custer

Tom, I have seen two types of pistons. In my yahoo photos is a photo of both types. The piston with the o rings installed should be correct for your car. The other piston has cup type rubber rings, they are similar to some rubber cups used in clutch or brake cylinders. The early type with the cup seals was used in 68 and 69 cars. I just looked in my box and I still have small o rings from the repair on my 74 so I think the bigger rings are correct for your car, if not they will not go into the cylinder.

Clifton Gordon

Clifton, yes that is indeed the type of piston I have. One end of the piston has a small dimple in the center. What has me confused is that the directions say to use the smaller of the O rings for the dimpled piston. Maybe the dimple is not actually the machined dimple that they are describing. Perhaps the dimple is from the lathe that turned the piston? I'll try the larger O rings and see what happens!

Many thanks for your help,
Tom Custer

After doing the research on this subject through this site about three months ago, I simply replaced the broken sensor with a stainless steel bolt. No problems at all with the car stopping, yet.
1977 MGB
randy olson

This thread was discussed between 22/07/2006 and 31/07/2006

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