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MG MGB Technical - 78B brake failure switch
|I think the actual name is the pressure differential warning actuator or something? The little plastic bodied thing that screws into the bottom of the master cylinder and has a wire going to the brake failure lamp.|
I was trying to bleed my brakes and nothing was happening except a small leak out from around that switch. So I was checking I had indeed unscrewed that switch by the 3.5 turns it says in the manual. To check I went to tighten it then loosen it again. Only it wouldn't tighten. Turns out the switch had broken and the thread had sheared right off. Had to pull off the master cylinder and extract the broken half of the switch.
None of that is a problem and I have a new switch arriving today I hope (bloody expensive if you ask me for a thing that isn't even a switch but a bit of plastic with a metal pin). The thing I don't understand is should there have been fluid leaking around the broken switch at all? It seems to me looking in the manual that there little shuttle piece has o-rings on it so there shouldn't be any fluid getting into where the switch lives? It is hard to tell from the diagrams and I had it all rebuild by a shop so I never saw it all apart myself.
|Simon, the switch should be dry, you are correct the seals should keep fluid out of this area. Time to rebuild the master.|
|It supposedly is rebuilt! Oh well, I have a new seal kit at home so I will redo it myself and do it properly.|
|OK, I was looking through the archives. It looks like since I have the master cylinder off the car I should be able to simply remove the little pressure warning shuttle and make sure the o-rings are OK and replace if necessary. Then reassemble it all with the new switch in place. I shall have a look into it when I get home later.|
I just saw your thread after posting a similar one about the pressure differential switch. Please let me know if you have any insights about replacing a broken switch with a regular bolt. Post your response in the other thread titled "Pressure diff switch".
|Ah, just discovered a little trick. There are TWO different sorts of shuttle/switch combinations. I will post a pic on my web site later. One uses the non switch switch which is just a simple metal pin. That shuttle just has a single cut out in it. I guess when it moves the end of the cut out contacts the metal pin closing the circuit.|
The second type uses a real switch. It's shuttle has the cutout too but with a ring in the middle. The switch plunger rests on this ring in the normal state. The switch is a NC type so since the plunger is down the switch is off. If the shuttle moves the switch plunger can pop out closing the circuit. It is this sort that is shown in the manual.
Now I understand why you have to unscrew it. If it is in place and then shuttle moves you can't then get it back into the central position. With the first kind it wouldn't matter.
Pictures make it a whole lot clearer!
Post the link to your web site. I'm interested in seeing the pictures of the switches, and understanding how the shuttle works.
|Haven't posted it yet but will in a few hours. Just trying to bleed the brakes now.|
|OK here are the pics. The mysteries of the PDWA solved!|
As i said I couldn't get the newer type to not leak in my master cylinder so I will just have to use the original I think.
I also had a problem where I wasn't getting any fluid into the rear brake line. Pumping the master with the pipe removed showed there was no fluid coming through but I could feel air moving about. In the end I stuck a bit of clean plastic hose in the outlet and sucked on it until fluid started coming out (use a LONG tube so you don't suck brake fluid). Once I had done that it would pump though fine. I guess I had an air lock in the master cylinder.
Only now I have run out of brake fluid so I can't finish the job until tomorrow when I can get some more!
|Simon. While it seems that your problem is solved, let me point out that Danny Wong was kind enough to write up the procedure for bench bleeding the master cylinder which, depending on circumstances, might be a good thing. He was kind enough to allow me to post it on my website, www.custompistols.com/ under the MG section, then click on articles. The articles written by other contributors are listed under their names. |
Danny and I had discussed this matter, off line, before he wrote his article. Many times, but not always, you can install a new master cylinder and do not have to bench bleed it. However, there have been enough people who have had problems similar to yours that it seemed to be worth while to consider this procedure. Danny writes quite well and I use his article as a guideline when installing new master cylinders on my cars.
This thread was discussed between 26/04/2006 and 30/04/2006
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