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 its 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.
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:
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.
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Copyright © 2003 by Daniel Wong.