How to bench-bleed a master cylinder?

[jamie]

Sorry if this is way too elementary for most folks, but maybe you can give me some clarification...

I know that bench-bleeding a M/C means getting all of the air bubbles out of the cylinder before putting it on the car. Well, I pumped mine a couple of times until it was spurting brake fluid and thought "There! Now I just need to hook up the lines and bleed at the wheel cylinders, while keeping an eye on fluid level in the M/C, and I'm good to go...". Well, I got to thinking and realized that without lines hooked up, then it's probably just drawing more air in when you pull the pedal shaft back out (as if you were releasing the pedal). So, I figured if I hooked up the brake lines and pumped it a couple of times, then opened them back up with the pedal still pushed down to let air out, and then screwed the lines back on before I released it, this would keep air from entering the master. But again, I started thinking about how my brake lines are dry right now, meaning air could be drawn in from the lines!


Now, am I over-analyzing this? Is the only way to bench bleed to submerge the openings in brake fluid while pumping it a couple of times? Even then, you have the same problem when hooking up the lines...

Somebody set me straight here...

http://www.jalopyjournal.com/ubbthreads/images/graemlins/confused.gif

[Hot Rod To Hell]

bend some lines in a "C" shape to go from the ports into the reservior. Then when you pump it it is really just pressing out air, and drawing in fluid. it's a LOT easier to pump it that much once it's bolted into the car.

[Fat Hack]

Hot Rod To Hell pretty much nailed it. There are a few applications where it is easy to "bench bleed" the master once it is mounted on the vehicle though.

68-72 Ford Econoline vans come to mind. They have the master mounted "sideways" and worked via linkage that can be "pumped" from under the hood with a big screwdriver in the bellcrank area for leverage and the hoses hooked up as described.

(Just remember to TIGHTEN your hard lines once you complete the installation after bench bleeding...or you'll un-do all of your work! I learned that the hard way on the first one I ever did myself!)

Keep gently working the master cylinder until you see no bubbles in the reservoir as you pump it...then it's ready to install.

From there, bleed your brakes, starting with the furthest wheel from the master first and working your way closer...add fluid as needed, then cap it off and you're DONE!

[poorboy]

this may be a really dumb-assed question but,,,, Do you HAVE to bench bleed the M.C. before installing it?

[Anderson]

Its best to do it to brand new ones

[four-thirteen]

[ QUOTE ]
this may be a really dumb-assed question but,,,, Do you HAVE to bench bleed the M.C. before installing it?

[/ QUOTE ]

I never have, and never needed to. The system won't let air in, because it's sealed. open bleeder, push pedal, close bleeder, let pedal up. works every time. Dave

[Lionheart]

Any new master cyl or any master cyl thats ever got real low on fluid must be bench bled. Do it like HRH or FH said. Make sure you get every last air bubble out of the master cyl. http://www.jalopyjournal.com/ubbthreads/images/graemlins/laugh.gif http://www.jalopyjournal.com/ubbthreads/images/graemlins/laugh.gif