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MG MGB Technical - Brake pressure failure switch

I am having trouble bleeding the rear brakes after replacement of the shoes. After some reading, I checked the P.D.W.A. and found that the switch itself is shorted and the shuttle "O" ring(s) are leaking. John Twist say just put in a short bolt to stop the leak and forget about it. Now the question; What happens to the shuttle? Does it need to be removed? If it is blocking the rear brake line how does one "reset" it to center? Thanks as always, Tom
Thomas McNamara

Tom

Taking the shuttle piston out is dangerous---doing that connects both circuits together---If you have a failure like a blown hose you will lose ALL brakes with the shuttle piston removed.---

Best action to take is--Remove the switch-get a small screwdriver down in the hole the switch came out of and push the shuttle piston back to the centre so that the deepest part of the groove in the shuttle is in the centre of the hole (reset to normal) --Some aftermarket shuttle valves are spring loaded and reset themselves as soon as the switch is removed.
Then find a bolt the same thread as the switch and grind it to a roughly 45 degree point and screw it in the hole to hold the shuttle piston in place--not much more than finger tight is plenty to prevent any damage to the piston. This part of the shuttle doesn't have fluid pressure in it, there might be a little fluid in there that's got past the piston seals but basically it should be separate from the pressure circuits.
Now with the piston held in that central position, go ahead and bleed the brakes- both circuits will be operational
After you've got yourself a good pedal, remove the bolt and refit the switch -- done-

willy


William Revit

My understanding is that the unboosted dual systems need to be centralised after bleeding by redoing the opposite side to the last one done until the light just goes out. For boosted dual systems partially unscrew the switch and that allows the shuttle to self-centralise during bleeding.

Does the shuttle block the flow to the side that has lost pressure? If so I don't know how you would ever get fluid through one circuit to get air out without locking it in position as described first. If not and you aren't bothering with a switch just plug the hole as John Twist describes. If the O-rings are leaking it would need to be sealed as well as plugged.
paulh4

Tom. I am not sure what a "P.D.W.A." is. But, from the description of the problem believe that you are posting about the 75 and later dual master cylinder.

Yes, when the shuttle on the left hand side of the main body of the master cylinder shifts frontwards or rearwards, the small pin on the brake system failure switch rises upwards and blocks the shuttle from moving until the switch is removed from the body of the master cylinder and replaced again. The shuttle is pressurized by the front (rear brake-one line) and rear (front brake-two lines) sections of the main chamber of the master cylinder and the fluid actually flows through the shuttle from the main cylinder to the hard brake lines. If the rubber O rings in the left had chamber become old and fail to seal, there will be some slight fluid leakage which exits the cylinder through the brake system failure switch.

The most common method of "correcting" this problem is to remove the switch, put some electrical tape over the wire that connects to the switch, and replace the failure warning switch with a short bolt. As I remember it, the correct size is 5/16" UNF, but I would have to check since it has been several years since I last had to do that.

The problems with the poorly designed system for determining brake failure is well known and has been for quite a number of years now. Many people, including myself, have used the bolt (with a little sealing compound on it) to seal the hole in the bottom of the master cylinder without any problems. But, as Willi notes, it might be considered "deactivation of a safety warning system" in lawyer speak. Hence, any decision to take that step is a personal one and must be made by the individual who is taking that action. I replaced a leaking switch some twenty years ago and have never had a problem with it since. But, that is not a quantitatively valid decision, being a sample size of one.

Les
Les Bengtson

Les
PDWA =Pressure Differential Warning Assembly
As you say the later M/cyl's had the valve built in to the cylinder itself ,but the early dual circuit cars had the valve assembly seperate on the inner guard -- doesn't matter though, both types work the same.

willy
William Revit

The adventure continues. Taking instructions from the Haynes manual, I gravity bled the front brakes and proceeded to the rears, The drivers side rear bled nicely but the passenger side will not bleed? Next I will try to force fluid out with pressure from the pedal. We shall see what that brings. THANKS for all of the advice, once again. Tom
Thomas McNamara

With one flex hose feeding both rears and the driver's side bleeding OK it won't be the hose.

Pipe crimped over the axle? Wheel cylinder nipple blocked? Although usually fluid escapes from the threads in that case.
paulh4

When the switch on mine started seeping brake fluid, I removed the plastic shuttle and installed new O-rings. Very simple and inexpensive. The O-rings are a hardware store item.

Once the shuttle is reinstalled and the system bled, if the light is on, remove the switch and reinstall and it will go out until/unless there is a pressure differential between the circuits.
Glenn Mallory

Okay, after a lot of attempts, the brakes were/are bled successfully! Thanks to all. Tom
Thomas McNamara

This thread was discussed between 19/05/2024 and 31/05/2024

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