MGB & GT Forum

I've followed and found and read a few threads regarding various difficulties with the bias block. One was mine about the brake light staying on and draining the battery.

The car is a 1974 MG-B and as far as I know the only thing that's been replaced is the rear wheel cylinders and rear shoes, which I did maybe 5 years ago. Perhaps the M/C but I don't know that and, at 85K miles, I have to suspect the front pads have been replaced. Well, the light has come on once or twice more but I caught it and did the toggle of the little piston, which turned off the light but I know didn't rectify the problem. Now....both front and rear brakes work fine and no leaks at M/C or wheel cylinders nor hoses nor fittings nor can I find any reason for the brakes themselves to toggle the switch....unless I'm overlooking something. Soo...in one of the threads I read where the bias block can be rebuilt. Other than the pain of bleeding the brakes, (a chore I NEVER learned to like), could/would rebuilding, or replacing the block eliminate the piston triggering the light? I'm thinking replace the darn thing, if that is the difficulty.

As usual, I am grateful for any insight.

I know you said you did the toggle of the little piston, but have you taken the "block" apart and cleaned it?

The piston inside has two small "o" rings on it.
The piston is designed to slide from one side to the other if there is a brake failure (front or rear),
When the piston moves, it activates the switch causing the light to come on.
Sometimes due to dirt and age, the piston can get stuck, and/or the "O" rings break down.

On my car, the piston was stuck to one side and would not let me bleed the rear brakes.

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Oil leak?? What oil leak? That puddle under the car is just sweat from all that horsepower!!

Law of Mechanical Repair - After your hands become coated with grease, your nose will itch and you'll have to pee.

It really isn’t a “bias block” as in being able to adjust or “dial-in” particulars regarding the balance between front and rear braking. It does, however act as an activator for an electrical switch which in turn activates a dash light indicating a differential pressure between the front and rear braking circuits.

Should you decide to retain it for that function, then I would remove and disassemble it to rebuild it ( new seals and copper washer), and perhaps renew the plastic switch as well.

Some folks remove just the plastic switch portion and install a very short bolt and sealing washer- which then turns it into basically a junction block for the brake lines and master cylinder feed. There is a lot of info about this unit on this site if you do a search. And yes, rebuilding it properly, bleeding the system and centering the internal shuttle valve will restore function (no light under normal conditions).

It is called a PDWA (pressure differential warning activator) in the vernacular.

Good luck with your project and report back!

Brian W.

are you asking after the dash warning light? @ 0.75W or the rear lamp brake lights?
I can't imagine the dash light draing a battery.
B

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Life's most persistent and urgent question is, "What are you doing for others?"

Thank you gentlemen, I appreciate your help.

It was an older battery and yes, it did drain it. I suspect it was on 2-3 months before I tried to start the car.

What Brian said.

The function of the block (which appears to have been sourced from the same place Ford used on Mustangs of the era) is just to trip the warning lamp if there is a pressure imbalance. I recommend keeping it. If you clean it and change the O-rings once every twenty years or so it will work forever.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com