Practical Classics (UK)

Bleeding profusely


QI have a Spitfire 1500. I read the January guide on rebuilding the brake master cylinder and intend to overhaul or replace mine. I have read that it would need 'bench bleeding' before refitting. Is this correct, and if so, how? Greta Bear, via email

Ed says

A'Bench bleeding' is filling the cylinder with fluid in relative comfort on the workbench. It's useful if the cylinder is virtually inaccessib­le after fitting, but otherwise unnecessar­y. It's likely to result in spilled brake fluid everywhere.

The point of it is first to make sure that fluid can actually pass from the reservoir to the cylinder outlets and is not held back by seals which have stuck together during long storage - and second to avoid pumping air into the brake system during bleeding.

However, there's an easier solution. First, make sure you can physically blow through the fluid inlet stub(s) of the cylinder and have air come out of the outlet(s). On some designs you feel little resistance; on others you might need to use a tyre pump or low pressure air. If you can't blow through at all, it'll be a pig to fill with fluid and may need stripping down and checking for stuck parts. A few, though, are just naturally awkward. Fit the empty cylinder to the car and make sure there is a clearance between the pedal (or servo) pushrod and the piston of the master cylinder. Do this by pressing the cylinder into place by hand and then releasing it. If it springs back, either make the pushrod shorter by adjusting it or add shims under the cylinder mounting flange. Without pushrod clearance, the system can't be bled easily and the brakes may bind when hot.

Fill the reservoir with fluid, with the outlet union(s) left loose.

Wait for fluid to run out of the unions. You may need to gently pressurise the reservoir and/or 'tickle' the pedal a little. Be patient. Then, preferably with an assistant to tighten and loosen the unions between pedal strokes, bleed the master cylinder through its own unions. When done, press the pedal a number of times with the unions tight to burp any air out of nearby pipework and into the reservoir. Check that the pedal is firm and if so, carry on and complete a full bleeding of the system, furthest to closest point, which will benefit from the change of fluid.

 ?? ?? Bleed master cylinders through their unions.
Bleed master cylinders through their unions.

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