MGB & GT Forum

Hi all, i recently rebuilt a RB 74 mgb. I noticed that the switch in the proportioning valve was leaking through the middle. I swapped it out with another used one that i had kicking around. The part itself looked like it was fairly new and it didnt show any signs of leaking. Next time i was at the car i noticed that one was full of brake fluid in the centre, ok, it must have been bad, swapped it out with another used one and same thing happened. I checked the forum for answers and the only thing i can think of is maybe i have to centre the valve. Is this the case? Are the switches that i swapped out now unusable or will they seal again once the valve is centered? Never run into this before in my 30+ years of working on mgbs

Mark Lawton

It is not a proportioning valve. Is is just a monitor to detect if the rears have lost pressure and you can't tell by driving. A pretty useless device which is why many replace the switch with a bolt.

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Cogito ergo sum periculoso

Mike,

Yep just eliminate the switch. You will need a very short 3/8-24 bolt and a copper washer for under the head of the bolt. That will seal op the leak. That device has no effect on the brakes effectiveness.

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CAUTION!!
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Or another way to fix it, is to fix it, by replacing the seals that are leaking on the shuttle piston. The switch should not "see" any brake fluid.

Fix it. Why give the legal/insurance system any opportunity, in the event of an accident, to say you reduced the safety of your braking system.

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It is easier to fight for one's principles than to live up to them.

When you worship power, compassion and mercy will look like sins.

PeterC has it.

Do the repair, not difficult or expensive. (below) the O rings fit to the hour glass shape shuttle.

You also will not have any disconnected wiring under the bonnet if you repair it properly.

B

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

As the guys have said - cheap and easy to fix - the kit is readily available (although you might be able to simply match the O-rings).

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

It does "detect" pressure loss on front or rear brake circuits. Where the wired connector screws in should be dry. If not the o-rings metioned need to be replaced and the switch will probably have to be replaced since the brake fluid eats the contacts quickly.

If I'm not mistaken, the H-block was "borrowed" from a mid-sixties Mustang.

I hope it's just 3 consecutive valves and hot cracked brass!

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Basil C. Adams
1956 MGA Coupe (Show Car)
1957 MGA Roadster (Driver)
1958 MGA Coupe (Racecar)
1959 MGA Coupe (unrestored)
1960 MGA Coupe (unrestored)
1960 MGA Roadster (Driver)
MKIII Elva Courier (E1056)
1967 427 Cobra
1972 Alfa Romeo Montreal
A coupla late MGBs
1960 Austin Healy BN7
More Cars than Brains

Mark,

The correct nomenclature for the item in question is: Pressure Differential Warning Assembly (PDWA).

The intention of this device is to warn a driver if one of the two brake circuits looses fluid and therefore pressure. The light assembly (which also has a "push to test" feature is located on the dash just to the left of the headlight switch (as you already know of course!).

Another name for this type of device is "shuttle valve". The little sliding portion (shuttle) of the PDWA with the two rubber packings normally sits centered. If fluid is lost from either brake circuit, the pressure from the good circuit will push the shuttle towards the side that lost fluid which inherently pushes up on the contact switch plunger and provides a ground to illuminate the light on the dash.

As mentioned previously, you can probably find the correct size packings (O-rings in layman's terms) at your local rubber hose dealer or anywhere that sells hydraulic system repair items.

Clean the contact switch with some electrical contact cleaner if dirty/wet. Key on, with switch connected, push up on the plunger. Light on dash should illuminate.

It is a very simple device as mentioned above...

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Those who confuse Burro and Burrow don't know their @ss from a hole in the ground...

Many thanks, I’ve ordered the missing parts and rebuild kit.

I want everything safe, sound and correct.

I’m also going to try and track down missing smog equipment.

Make sure to get EPDM o-rings rather than Buna N or Viton. EPDM is brake fluid safe, the others will fail quickly.

Although not mentioned yet, there are two version of kits. Some have a lip seals and some just an O rings. Make sure you purchase the correct one.

Mine had a bolt, which was way too long and someone made up the difference with a stack of washers. I bought a new switch and of course it leaked just like yours. I put a proper length bolt in with a copper washer but that is only there until the new seal show up. I also recommend new seals.

B

I’ve done the o-ring repair and it works perfectly. The most expensive part of that entire system is the plastic plunger sensor :-o

As mentioned; there are two versions of o-rings. Just be sure to re-center the shuttle before bleeding your system and check again after using a toothpick-type tool for the shuttle.

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There’s no such thing as coincidences; just unrealized connections.