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MG Midget and Sprite Technical - Brake Pressure Differential Switch

Was reading through the archives about people having trouble bleeding their am I.

I'm at the tail end of a 17 year re-build and doing the brakes now. Rebuilt the master cylinder (this could be the main problem too) and am now trying to bleed the whole system. I bough a vaccum bleeder (since I have zero help) but when I start at the back...get nada. No fluid. nothing. Same on both sides of the rear. So far I can only draw fluid through the front driver side. But LOTS of air, with no reduction in air after a LOT of pull through.

So...a few quick questions:

1. Where would you start?

2. If it's a buggered differential switch, can't I just remove the piston and allow balanced equal flow all the this system and using the differential as a simple connection point for all 4 lines?

If no to #2 (understanding that I learn by pics much easier than words) how do I test if my switch is ok / not ok / stuck in a position preventing any fluid from getting to the rear cylinders.

Maybe I'm being very impatient and need to re-fill the main reservoir a LOT before I get no air, but I don't think so.


Chris Elkerton

So if i understand you have a later midget with split braking system.

You now realy need an assistent.
The easiest way to check is see if fluid comes out all connections.
Tricky as you do not want to damage your paint.
But have a rag ready and someone in the driverseat.
Undo one connection and have the assistent push the pedal (slowley) fluid should come out.
Repeat on all connections and you know if the MC gives fluid.

If you have no way to find an assistent the get a pressure bleeding system.
These put fluid pressure on the reservoir and work better with empty pipes
Onno Könemann

The experience I have with dual brake system comes from my '72 Volvo:

When you bleed the brakes, first remove the springloaded switch from the hydraulic system. Also to reset the differential valve.

Don't know if it's any help for a Midget though.
Alex G Matla

sorry all...yes...I have a 1975 1500.
Chris Elkerton

Chris, the differential switch merely consists of a metal shuttle that will move one way or the other depending which brake line fails. You could remove it, but I feel it's an important safety feature in the event of a brake line failure. Without it, you will have no braking pressure at all.

I guess the best way is to see if the shuttle is centred first. This will allow some fluid through into the system as you begin bleeding the brakes. Having an assistant always makes things easier too.

Bleed from the longest line to the shortest was the way I was taught to do the job.

Bleeding the brakes is fairly easy compared to that pig of a clutch!
Clive Reddin

See Clive is in Toronto and I'm sure will go and press pedals and get it sorted. The easibleed is good for general use but won't help if there's a problem.

Yet again the lack of seeing is hard - when you say "alot" - how much do you actually mean as alot is subjective and it might not be to me!

This thread was discussed between 22/08/2011 and 24/08/2011

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