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MG Midget and Sprite Technical - brakes

Having bled the brakes and got the air out the pedal seemed fine, but as soon as the pedal is released the fluid returns to the m.c. it then takes 2 or 3 pumps to get the pedal firm again. do i need residual pressure valve in the line
PP Lawson

I,m no expert and would shy away from giving anybody advice about such an important safety aspect of the car, but I would suggest that a correctly functioning standard system should be perfectly acceptable. I don't think you should be looking at adding anything like that until you are absoloutley confident that there are no leaks or air. having to pump the brakes must mean you still have air in the system somewhere. I had a similar problem bleeding the clutch, pumped loads of fluid through, still a weak pedal, after sending loads more of fluid through the pipework a big bubble of air came out and all was then fine.
I'm sure more technical help will be along soon!

M Adams


Nothing magical simply you have not got all the air out of th system. When all the air is out of the system then the pedal will resit your foot when you press it. Normally you should not be able to press it much further than about 3/4 of the way down.

The only exception to this is someimes due to the rear brakes being badly adjusted. badly adjusted rear brakes can ive very similar symptoms to air in the brakes to someone not used to this kind of diagnosis so make sure they are correctly adjusted before bleeding out the ai in the normal manner
Robert (Bob) Midget Turbo


I had an unusual variation on this, drove me mad (3 times around the car bleeding each wheel) till I found it. I was sure all the air was out (as subsequently proved to be the case) and the rear brakes adjusted properly, but couldn't get a pedal.

In the end I tracked it sown to the calipers. The (new) seals were gripping the pistons too much, not allowing them to slip through and self adjust. Instead when the brakes were applied the seals were compressed axially by the piston as it moved towards the disc, when pressure was released the seal obligingly acted like a spring and retracted the pistons about 20 thou.

It didn't look much, but when I did the maths on the displacement volume over 4 pistons it accounted for most of the master cylinder stroke. I suspect that the culprit was rubber grease which was a bit old, had thickened over time and was no longer doing its job properly. It was simply fixed by pushing the pistons most of the way out and giving them a thorough clean to remove the offending "lubricant".

After which the brakes became as they should be.
Paul Walbran

Thanks for your help, problem solved, it was just could old air in the system.

cheesr Phil
Phil Lawson

This thread was discussed between 28/04/2012 and 30/04/2012

MG Midget and Sprite Technical index

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