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MG Midget and Sprite Technical - Brake Pressure Differential Switch

I thought I would start a new thread after looking in the archives while continuing to resolve the problem with bleeding my rear wheel cylinders after rebuilding the brake master cylinder...

The BBS archives indicate that the movement of the shuttle piece inside the brake pressure differential switch completes a circuit & activates the warning light in front of the driver. My switch has never turned this light on - and a quick check of the two wires (black w/white) that connect this pressure switch with the warning light on the dashboard, leads me to this question:

In the photo below, the tip of the switch can move inward (probably after being moved by the shuttle piece and brake pressure, etc.) - however, based on my multi-meter, I cannot get any continuity while pushing against the tip of the switch after removing it from the vehicle. I would presume that the shuttle piece exerts MUCH more pressure moving the shuttle around in the event of a pressure failure...

My question is: even if the switch has failed internally, but has NOT begun to leak, how likely is it that the shuttle piece is stuck in a position such that it would prevent me from bleeding the rear brake cylinders? The location of the pressure differential switch is such that possibly using a small mirror, I might be able to see the shuttle piece.

If my lovely assistant exerts force on the brake pedal - will I see the shuttle move? Do I have to induce a "failure" into the brake lines (i.e., by cracking open either the front caliper or rear wheel cylinder)? Has anyone experienced (or observed) a shuttle piece corroded into a fixed position preventing successful brake line bleeding?

Sorry for the long post, but this problem regarding brake line bleeding is important from a safety perspective...on the other hand, my wife has been seen talking to the local life insurance agent...

Thanks for any thoughts or observations...

Mike P.
Buffalo, NY

Mike Pelone

I had a similar problem. I concluded that my switch was bad (you can test it by hooking it up to the circuit and press the button). The light on the dash should light if the switch is good. (and you can check continuity too).

I had two problems. The first was that the switch was bad. I replaced it. my second problem was that when bleeding the brakes I pushed the pedal down too hard and forced the shuttle valve to move off center thus cutting off the flow to the rear. I ended up carefully taking a tiny screw driver and sticking it in the hole where the switch plugs in, I was able to push the shuttle valve back to the middle. Then I could bleed the brakes.
After it was all bled and my new switch was installed I was able to balance it (by opening the appropriate bleeder until the light just went out).

This is what my shuttle valve looks like (I replaced the o-rings):


R Harvey

Based on testing with my multi-meter, the switch is likely bad, however - that is the secondary problem. I can always replace the switch later...the immediate problem is to re-center the shuttle and attempt to get brake fluid out of the rear wheel cylinders.

Thanks for the photo of the shuttle piece - I have not seen that little bugger before.

I have owned 3 Midgets during the last 30 years and have never had this problem before. I always bleed brakes as part of my Spring safety checks prior to hitting the asphalt - maybe it was time to get another bite mark on my ass...Oh - well, live & learn!

This summer the car has not left my garage.

I will try and use a small mirror and a small screwdriver or pointed object to get in there and move the shuttle -
One last question: in your case, were the old seals and O-rings holding the shuttle piece off center - did you see any corrosion?

The kit is around $12.95 from Moss...I hope forcing the piece to re-center does not destroy any seals inside...

ps: has the Pangalactic Consortium found any new Life Forms lately?
Mike Pelone

On my brake pressure warning assembly, the seals on the shuttle had failed, allowing the master cylinder reservoir to drain through the switch. But in my case, the switch was still good. All it should take is a light press on the switch pin to illuminate the warning light.

BTW, the "official" bleeding procedure in the shop manual (Bentley - I don't know about Haynes) is to connect bleed tubes to both the front and rear brakes on one side of the car, and then bleed conventionally using the pedal. Since both sides of the dual circuit are then open, the shuttle will stay centered. The downside is that unless you have those fancy ball-valve bleeders fitted, you'll need a person at each bleeder to open and close them at the appropriate times, in addition to the usual Lovely Assistant working the pedal.

I've always just bled the brakes individually, but gently. So far, so good.

Gryf Ketcherside

I don't think that my o-rings were bad but since I had to remove it to find out I replaced them (enough trouble to take a look).

I am using stainless steel speed bleeders everywhere (including clutch). I really like them. For the clutch I use one of those gadgets that lets you suck fluid from the bottom (has a vacuum gauge on it and you just squeeze it to build up vacuum). It worked perfectly and completely solved my problem if air getting trapped in the line.
R Harvey

The pressure differential switch is obviously dead - so that item is now on order plus I will pop for the Moss O-ring seal kit. Better safe than sorry...besides, brass looks pretty "pretty" when you clean it up...

I put those SpeedBleeders on the front last year - that was the last successful brake bleed session. I am sure the new bleeders on the front calipers have nothing to do with my current problem...

So, here is what I did this morning:
I by-passed the pressure differential unit by using very short lengths of brake line that feed back into the reservoir. Front piston (front brakes) moves large volumes of fluid and NO air while the rear piston (rear brakes) seems to "push" air, then sucks fluid back up the plastic hose. Also, the pedal stroke is very short...12 pumps of the pedal and the brake fluid barely covers the bottom of a Snapple jar.

My theory? Pressure differential unit is probably OK - but when they resleeved and re-kitted the Master, the drain hole (?) in the back side of the reservoir is partially covered (or closed) so that very little brake fluid makes in into the piston cylinder to be pushed out by the brake pedal...

Anyway, that's my story and I'm sticking to it!

Mike P.
Buffaloed in Buffalo, NY

Mike Pelone

This thread was discussed on 11/08/2010

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