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MG MGB Technical - Pressure Differential Warning Actuator

Going to be bleeding the brakes on my '71 MGB after a long winter and for the first time since I have had the car. Haynes mentions disconnecting the Pressure Differential Warning Actuator if fitted. Question, do '71 B's have them?
Thanks
Ken Martin

It's the large brass thingy with all the brake lines running into it and the switch. There is a shuttle piston between the circuits in there that activates the switch if an issue occurs in one of the circuits. If it activates while bleeding the brakes you will have to re-set it (and as I have a '67 I can't give you the proceedure off the top of my head). Dis-connecting the wire will have no effect on either bleeding or tripping the shuttle. If you either power pleed or vaccum bleed (or are gentle with the pedal) you should have no issues as it is designed to engage when the brakes are actually in use (high line pressure) with no or little pressure on one of the circuits.

Mike!
Mike!

Hi Mike,
So I assume a '71 B has one, it's not clear in Haynes.
Ken
Ken Martin

Ken: Your 71 should have a brake failure warning switch located on the inner fender near the rear carb. It's brass and has five brake lines connected to it. You don't have to worry about it to bleed your brakes. If the warining circuit is working and the shuttle isn't frozen in place you may get a brake failure light on the dash indicator. It does not cut off fluid flow, it turns on the light to warn of a brake unbalance. In fact I removed the electrical switch on mine and installed short bolt and brass washer in it because it was leaking and I have no desire to remove the valve and replace the shuttle seals. My dash indicater light doesn't work anyway.

The Pressure differential warning actuator you asked about is in 74 1/2 on cars and installed in the master cylinder.

Good luck, Clifton
Clifton Gordon

When the light doesn't work properly, it's a simple job to remove and clean the switch contacts.
Dan Robinson

Ken,

I have not worked on an MGB with a shuttle valve operating a light which indicates that a partial hydraulic failure has occurred. However, I have worked on many Triumphs that have them. Sometimes, if you bleed the brakes slowly, the shuttle valve does not move to one end and activate the light. If you press the brake pedal after you close the bleeder it will certainly move. If it does move, on the Triumphs you remove the electrical switch on the valve body, insert an object such as a nail in a groove machined in the shuttle valve and bring the groove to the centre of the hole.

Mick
Mick Anderson

Ken,

If my "old rusty nail" method of centering the shuttle sounds a bit too barbaric, I will give you the official method.
From your Workshop Manual determine which brakes are on which hydraulic circuit. Front brakes on one circuit and rear on another circuit, or one front brake and the diagonally opposite rear brake on each circuit.
If you don't know which way the shuttle moved when the light came on, do the following.
With two people, one person opens a bleeder valve while the other slowly depresses the brake pedal. Stop as soon as the light goes out and close the bleeder at the same time.
If the light does not go out, repeat the procedure with a bleeder of a brake in the other circuit.

Mick
Mick Anderson

My Haynes says to remove the leads "and unscrew it 3 1/2 turns to bring the plunger clear of the piston" when bleeding. The Workshop Manual doesn't have additional information for bleeding with a pressure failure switch, but somewhere I have read that if *after* bleeding the light is lit, to recentralise the shuttle valve you open a bleeder on the circuit you *didn't* bleed last, press the brake pedal slowly until the light goes out, then close the bleeder. Either the Haynes info implies that the shuttle will return to the central position automatically, or the information is simply to prevent operating the switch for some reason and they have missed out the bit about recentralising the shuttle. Unscrewing the switch altogether *should* allow you to manually centralise it as Mick sez. Haynes is misleading anyway as it says after reconencting the wiring turn on the ignition and operate the handbrake to check the warning lamp glows, but that only applies to 77 models and later. On the earlier models you operate the test switch which checks the wiring and switch connections (but not the switch contact), and even doesn't apply to the 68 model.

Surely the thing on the inner wing with five (four?) brake lines in it contains the brake light switch, not the pressure failure switch. Isn't the pressure failure switch part of the master cylinder?

"If you press the brake pedal after you close the bleeder it will certainly move"

Eh? That would indicate a problem in the hydraulic system. Did you mean "after you *open* the bleeder"?
Paul Hunt

"Surely the thing on the inner wing with five (four?) brake lines in it contains the brake light switch, not the pressure failure switch. Isn't the pressure failure switch part of the master cylinder?"

2 different systems.

The direct acting servo master cylinder has the pressure failure switch on the master cylinder.

The earlier north american spec cars with dual circuit brakes had the pressure failure switch in a 5 pipe brass fitting on the left inner wing with a mechanical brake light switch on the pedal box cover. My Feb '74 ex california car is like this.

David
David Witham

Fairy nuff ...
Paul Hunt

Paul,
I had a disconnect between the brain and the finger. Yes, if you press the brake pedal after you *open* a bleeder the shuttle will move.
However, if you press the brake pedal very slowly when bleeding you might be lucky and not move the shuttle.

Mick
Mick Anderson

Wow! seems like I've opened a can of worms here. The main thing I can understand is that, according to Clifton, the PDWA was not fitted until '74½ years. Therefore it doesn't apply to my '71. That's all I need. Thanks guys.
Ken
Ken Martin

Clausager he say: dual-circuit braking fitted to North American cars in 67, brake pressure failure switch fitted in 68. The pressure failure warning actuator *and handbrake warning* was fitted in 76, this was a different system to earlier, but the 71 still has a gizmo that needs resetting if bleeding the brakes has upset it.
Paul Hunt

Nothing's ever easy is it. I'll just do a slow bleed and cross my fingers I guess. Thanks again.
Ken
Ken Martin

Ken,
Push the dash warning light to see if works. If it doesn't just bleed the brakes and forget it. If it works or you want it to function as it was designed, bleed the brakes rear to front. You can turn the valve switch out slightly to try and keep the valve in the center position but its easier if the light is on when you are done to just go to the oposite side of the line and bleed the pressure back until the light goes off.

LaVerne
LED DOWNEY

This thread was discussed between 21/02/2005 and 24/02/2005

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