Bench Bleeding the Master Cylinder

by Daniel Wong

Les Bengtson's Classic MG Pages: Articles:

Sometimes after installing a fresh brake master cylinder into an MGB (or any other car) the pedal will often repeatedly go to the floor and it may take many cycles of pedal pressing and bleed screw opening & closing before reservoir fluid even begins to slowly trickle it's way down into the master cylinder.

Priming the master cylinder with fluid before installation eliminates much of this.

Although there are several ways of doing this, "bench bleeding" the master cylinder is the "classic", tried and true method and it requires a minimum of new tools.

You'll need:

2 - short lengths of steel brake lines (about 2"-3"), each with flared ends and threaded fittings. These can be had off of a wrecked car, or by cutting a new pre-fabricated line.

2 - 12" lengths of 3/16" i.d. vinyl plastic hose (ie: pet fish aquarium air hose) and hose clamps.

2 - plastic caps or rubber stoppers (for plugging the outlet ports).

1 - pair of needle nosed pliers, or non-locking hemostats.

1 - empty container, at least 1 pint or more. (ie: mayonnaise jar).

…and lots of rags.

To bench bleed the M/C:

Mount the M/C on a sturdy workbench vise - by the flange - not the cylinder body!
Install the short steel brake lines onto each outlet port.
Install clear vinyl hoses onto each end of the short steel brake lines and use hose clamps to insure a good seal. Route the free ends of the hoses into an empty container.
Fill the M/C reservoir with fluid.
Manually push the M/C piston all the way home - and hold it in.
Pinch either one of the vinyl hoses shut with a pair of needle-nosed pliers, or non-locking surgical hemostat.
Slowly release the M/C piston, allowing fluid to be drawn into the cylinder.
Release the hose (pliers). Keep the ends of the hoses immersed in fluid to prevent air from migrating back into the cylinder.
Repeat Steps 5 - 8 until all the bubbles are gone. Occasionally tap the M/C lightly to help dislodge any trapped bubbles. Be sure to keep the reservoir filled.
Go to the other hose and perform Steps 5 thru 8 for that hose.

You're ready to go after all the air has been expelled from the cylinder.

Remove the steel lines and plug the ports with protective plastic caps or rubber stoppers to minimize fluid loss. Wrap the cylinder in a rag before carrying it to your car for installation as you do not want to drip fluid anywhere on your car.

You will still have to bleed the entire brake system after you install the M/C but bench bleeding the M/C prior to it's installation saves you from much of the initial dry pedal stomping and empty (air) bleed screw turning.

I have seen other folks route the plastic hoses back into the M/C reservoir when bench bleeding their M/C's - but I do not recommend this as this recycles contaminants (grit, swarf, rubber bits) back into the cylinder. This is not good.

Actively pulling (vacuum) or forcing (pressure) fluid into the brake system are alternative methods to bench bleeding, but they require either an air compressor or a source of vacuum. Personally, I use a Mity-Vac to pull fresh fluid into the system as it eliminates the need for bench bleeding altogether and it minimizes dripping fluid onto the car. I then follow this up with a few strokes of conventional pedal bleeding as I find that it helps seat the seals, and this action is able to swish the fluid through the lines more quickly than a Mity-Vac to help further dislodge any remaining stubborn bubbles.

This monograph may be reproduced only for non-commercial use without other permission of the author. Reproduction for commercial use only by written permission.

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