How long to bench bleed master brake cylinder?

[JLawrence08648]

I just installed a master brake cylinder in an Acura Vigor.

Instead of bench bleeding in a vise and having to push the piston manually, I installed it leaving the brake lines off, having my wife pump the pedal while I used the enclosed plastic hoses to recycle the brake fluid back into the master cylinder. I immersed the ends in the brake fluid and noticed an awful lot of small bubbles coming through the hoses, the bubbles never stopped. I thought the bubbles may have been a result of the circulation of the pumping of the fluid back in to the cylinder.

Yes?

If not, why so many bubbles? They didn't seem to cease. I was just going to attach the lines and bleed through the bleeder screws through a hose into a clear plastic container.

 

[andyd]

Your enclosed plastic hose fittings probably aren't perfect. Use a little BF in your clear vessel to create an airlock. Watch the level in the reservoir.

 

[expat]

I have to ask because before I came to North America I had never heard of bench bleeding.

What is the point?

It just seems like good chance to get brake fluid splashed everywhere.

Why not just send the air down the line and be done with it when you bleeds at the wheel cyls?

 

[E150GT]

Perhaps the hoses are letting air in and also when I bench bled I had to manually pump it very slowly. Go fast and bubbles seem to come out of everywhere

 

[JLawrence08648]

I also don't understand it. When you install the master cylinder with fluid in it after bench bleeding you are letting air in. I'd rather loosen the nuts at the cylinder, press the brake pedal, close the nuts, let go, press again, open the nuts, repeat, then bleed through at the wheels.

 

[eljefino]

You're supposed to do it on a vise so it's level. When it's on the firewall it isn't level, and there's a bubble at the far end (headlight end) you won't get out.

This depends on the design and where the fitting lines are tapped. Some MCs need it, some don't.

While the fittings might be cheezy and introduce bubbles, if you're given instructions with your new MC to bench bleed it, don't skip this step.

You could also get a dishpan and a gallon of dot-3 and just manipulate it in the "swimming pool" but this is wasteful.

 

[AVB]

eljefino nailed it.