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MG MGB Technical - Brakes

Hi can anybody help please ? Brakes - Bleed as usual, pedal hard - NO BRAKES. Next day, pedal nothing at all. New master cylinder fitted bleed as usual, no air coming through, NO BRAKES. HELP?

Malc, a little more information might help. Is this an early car with the single line system? If so, then I suspect a small leak somewhere, perhaps in a fitting or hard line. I would think you would have noticed a leak at a wheel cylinder or caliper. Try placing sheets of paper under the car overnight (after bleeding), along the path of the brake lines and see if any spots appear.If it's a later car with the dual line system, then it's very unusual for both systems to go bad at the same time. Verify that neither system is working by placing the car on jack stands and have an assistant press the pedal while you try to rotate the tires. If indeed both systems are down, then I'd suspect the master cylinder even though it's been replaced.
Bill Young

When bleeding the master cylinder, care must be taken.
Don't let the pedal go all the way to the floor when you open the bleeders. Use short gentle strokes with the pedal. If you bench bleed the master cylinder, take short gentle strokes when pushing in the plunger. If you push in the plunger too far the seals can be damaged. Using gentle strokes prevents the air pockets from turning into a lot of small air bubbles.

Thanks for the quick reply's. My mg is a 1970 Bill and i can see no leaks. All brake pipes are almost new BUT THE CAR HAD NO BRAKES WHEN I BOUGHT IT.

Have you solved this problem? Hard pedal and no brakes equals frozen brake pistons. Maybe.
Ronald Milostan

Problem not solved yet, but still looking.pistons not seized, Recap - bleed brakes, pedal hard. NO BRAKES AT ALL. Next day pedal soft, fitted new master cylinder, same again BUT now I have noticed brake fluid being forced out the top of master cylinder cap when engine running. HELP PLEASE!

Malc, fwiw here are my thoughts about your puzzle...

When you're bleeding, if you're getting fluid out of all four bleed screws then the lines must be good and the flexible hoses can't have collapsed internally. So after you've bled then if you have no brakes at all - at all four wheels - then either both the cylinders and both the calipers are seized, or there's a problem in the master cylinder, or there's a problem in the servo (if you have one) that only shows up under pressure.

For the pedal to be soft the next day, then either air is getting in somehow, or there's a problem in the master cylinder or servo. If air is getting in then you'd see it next time you bleed.

For fluid being forced out while the engine's running it must be the servo.

How about making up a short pipe to bypass the servo so you can either pinpoint that or rule it out?
If the servo passes - then the new master cylinder must be suspect
Miles Banister

Fluid being forced out the cap would not normally be from the servo. Problems in the servo usually result in the engine burning brake fluid as the servo runs under vacuum. This really sounds like an assembly problem in the master cylinder, may be a cup seal in the wrong dirrection.
John H

Thanks but Miles and John were the nearest. When I bought the car there were no brakes, so as you have said seized calipers or bleed them. But calipers were ok and it bled ok, master cylinder was new so then I looked at the SERVO and found that the brake pipe from the master cylinder had been connected to the SERVO the wrong way round. So I re-fitted the pipes as they should be, brakes locked on solid with engine running.
Time to change SERVO. SERVO replaced - problem solved.
Obviously pipes fitted the wrong way round had damaged the SERVO. Thanks again, Malc...

This thread was discussed between 07/07/2006 and 12/07/2006

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