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MG MGB Technical - brake master cylinder

Pulled, rebuilt, re-installed MC and pedals, replaced a piece of brake line. Now there is no brake fluid in the line of this single line system; it is simply not coming out, even with a bleeder/ suction device. Ideas, please...
L E F Page

Your M/C needs bleeding. Try disconnecting the line from it at the 4 way junction, hold your finger over the end and have someone slowly pump the pedal. Your finger will act as a one way valve, hopefully causing suction in the M/C which will fill with fluid.

Herb
Herb Adler

If there is fluid in the master then a suction device on the lines should pull it through, unless there is a major air leak elsewhere.

However one cause of it building up a vacuum in the line and not pulling anything through, is if the master piston isn't coming back far enough for the pressure seal to clear the bypass port that comes down from the reservoir. That could be because of incorrect parts or incorrect assembly.

See the attached. The piston has come back just far enough to clear the port which allows fluid though. If that doesn't happen then none of pumping, pressure bleeders on the master, or vacuum bleeders on the lines, will get fluid through.

Paul Hunt

Could the brake light switch be preventing the pedal from returning fully?
Dave O'Neill 2

Dave, the brake light switch, on single circuit systems, is hydraulically operated and located at the four way junction. It should have no effect on the pedal returning from the floor. RAY
rjm RAY

good point, I missed the reference to single line.
Dave O'Neill 2

When the brake pipe forms a loop higher than the master cyl, the air bubble goes back and forth, not bleeding correctly, if at all. The method described by Herb should help. Personally I prefer the pressure bleed system which pushes the fluid through and the air with it. Moss do a cheap kit for this very purpose.
Jim
jim soutar

In case some are confused, although the US changed from a hydraulic switch when they got the dual circuit, the UK changed to a pedal operated switch with the start of rubber bumpers even though the single line circuit continued until the end of the 76 model year.

A vacuum should pull air through just as pressure should push it through, although I prefer the pressure systems as well. Especially with the small bore of the brake lines, although both can have problems moving air down the larger bore clutch line, which is why so many seem to have problems bleeding the clutch.
Paul Hunt

Paul, thanks for the clarification. RAY
rjm RAY

This thread was discussed between 16/12/2014 and 18/12/2014

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