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MG MGB Technical - Brake Failure Pressure Switch

Just a quickie for you. I have been struggling to stop a slow leak-down from my master cylinder to the Four way splitter for a year or more. It is worst in the winter, and it actually drains the res. if not topped up. No ammount of spannering the nylon switch works, and it is leaking out the "plug" end, rather than the screw threads. I had a spare, which I replaced today, but I am wondering how many of you have replaced the switch with a brass plug and are motoring on, happily, with no warning circuit in operation- and if so, what is the plug thread/size?
Ken R
Ken Rich

I don't know the plug size Ken, but I have the same problem. I even tried to make a gasket without success. I would plug it if I could find the right size plug.
Jerry

Jerry, I have had a massive brain wave! I have a spare four-way splitter in my spares box- i will take it down to the auto parts store, and buy a plug to fit. Then I will get back to you with the size, on this thread. Tomorrow, Friday,PM, PST
Ken R
Ken Rich

Ken and Jerry, My 68 GT has a plugged brake failure switch. A 3/8" 24 NF bolt will fit or it does in my spare and the one in the GT. You need a bolt with threads to the bolt head about 3/8" to 7/16" long, you may have to cut a longer bolt to the desired length. Use a brass washer(Same as as used between the front brake flex hose and caliper)on the bolt for a seal.

The reason for the leak is the seals on the shuttle valve piston are defective. You can rebuild it with a kit, maybe. I say maybe because I ordered a kit and the seals were not like the ones in my spare four way
valve. I'll check the one on the car to see if my kit will fit it when I do my next fluid change. If not I'll have to try the alternate kit for early cars. Hope this helps. Clifton
Clifton Gordon

All that are in the Lockheed kits (for a 72B)are 2 different sized pairs of o-rings and a small copper washer. These are relatively small o-rings, about 1/8 ID. Once apart, you may be able to find a local hardware or industrial supply store with the correct size o-ring. The cost of o-rings is really next to nothing, but they get $6 0r $7 dollars for a kit. Moss lists a more expensive kit for the earlier cars,and I am not sure what is included. I had the same problem last spring, but found it to be a relatively easy job, had it out, replaced the o-rings and installed in about an hour. It stopped leaking through the switch after that. Since I had to bleed the system after install, I learned I needed to "centralize" the switch to cancel the failure light on the dash after that, this was accomplished by opening one of the rear bleeders and slowly depressing the brake pedal until the light went out and the shuttle was centered.
Joe

When I switched my '67 over to the later dual circuit system I did just as Clifton suggests. Works great, and you can usually tell when one side of the brakes starts to go out so I wouldn't worry about the lack of warning light.

-Jared
Jared Snider

Thanks blokes for your help. One last Q: with the switch out of the circuit, is it still important/possible to "balance" the piston shuttle to the centre position? i.e. do the brakes work as efficiently? Merry Christmas.
Ken R
Ken Rich

Ken, It isn't necessary to position the shuttle with the switch out of the circuit. Each end of the the shuttle piston extends beyond the seal and the piston will not shift enough to affect brakes. The main function of the four way valve is to turn on the brake failure light in event of brake failure in one of the the circuits. Regards, Clifton
Clifton Gordon

OK, Clifton, thanks. Just as I had hoped. This thread can go to bed.
Ken Rich

One comment on this: The early units, as in my 68 GT came equipped with cup type seals similar but much smaller to those often used in master cylinders. They fit over the shuttle, sealing against the shuttle rod and the bore of the brake switch manifold. At some point, and I don't know where in time, this manifold was simplified somewhat and the use of O-rings came into use. When I attempted to fix my switch manifold a few years ago, no supplier seemed to be able to dig up the cup type seals, only the O-rings. If they are available today, I wouldn't mind knowing the source! FWIW.
Bob Muenchausen

Bob, My spare is from the 68 GT, I installed a manifold from a 71 parts car and planned to install a kit in the manifold from the 68. The plan was to reinstall it in the 68 when I next change the brake fluid. The kit I have is Moss #181-995 and has O rings, consequently I could not install it in the manifold from the 68 for the reason you mentioned.
Based on the car numbers listed in the Moss catalog it appears the change was made sometime around the end of the 69 model year. Moss does list part #181-985 for RD 138401 to 167815 and GT 139472 to 167815. It lists for almost 3 times the price of the O ring kit so perhaps that is the one we need. Regards, Clifton
Clifton Gordon

Interesting thread, as the PDWA splitter on my 72 US spec BGT is also suspect. I am in the middle of a full brake system rebuild, and I was thinking of replacing the PDWA splitter with a straight connector ( for the rear), and a standard three way (for the front).

Any comments?, anyone tried the same?

Look forward to responses,

Paul
Paul

Paul,
During the full "midlife" restoration on my 72 US specs B roadster which is now almost finished I also discoverded a leaking PDWA splitter, apart from a heavily leaking tandem master brake cylinder which wetted my shoes during braking.
I took the splitter out, dismantled it and went to my rubber O-ring supplier with the shuttle valve. Within 2 minutes he provided me with 2 new fitting oil resistant O-rings and it hasn't leaked since I put it back in place and pressurised the brakes a 100 odd times. Any use?
Henk van der Ben

Yes Henk, of course your comment is of use- it is a valuable reminder to all of us that we don't have to buy a highly priced "kit" that is comprised of 4 O rings. I thank you on behalf of every guy who will make a mental note of your helpful post for that day, which will come, no matter what we do in terms of brake servicing. Happy New Year.
This is of course, the answer, new O rings/seals, as Clifton pointed out. I know it is the answer, because my new switch is already weeping fluid, which is dripping down to join the fluid from the original leak. Hope it lasts until I get back from AUS!
Ken R
Ken Rich

Thanks Henk and Ken, just a couple of thoughts; first I think we should all be as safe as possible when dealing with the braking system ; second if the PDWA is the cause of a (potential) pressure loss, why not remove it? It seems to be warning me of a problem it is causing!

By the way Henk, did you replace the seals in your tandem master cylinder? I have exactly the same problem (even down to the shoes!) but I cannot dismantle the cylinder, I cannot get the first circlip out, even with brute force and ignorance, so am looking at a new one at GBP140 (!). Look forward to your replies.
Paul
Paul

Paul, DON'T buy that new cylinder yet- check out Haynes, page 155( without PDWA) and p156,(with PDWA) for a clear description of removal and dismantling. Also, check out the archives. You can save a lot of money and have perfectly safe brakes.
Ken R
Ken Rich

Thanks Ken
Unfortunately Mr Haynes says "remove the piston retaining circlip".
I have struggled for many hours, as have three of our mechanics in the workshop. The conclusion is either that it is one of these US safety things and not intended to be removed, or a PO has forced the wrong circlip in. The only possible way is to cut the primary piston and depress it into the cylinder far enough to pass that (and the next) circlip, unless any of you have any better suggestions? I was told by MGOC that it was not possible to get a replacement piston, so have not cut it (yet?).
Any help would save alot of money, so thanks in advance.
Regards Paul
Paul

This thread was discussed between 19/12/2002 and 28/12/2002

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