I've been struggling with some leaking banjo bolts and think I've finally got that fixed.
Then, I was bleeding my front brakes and did the dumb move of letting the fluid get too low in the master cylinder and it sucked some air in.

I guess I need to bench bleed the master cylinder, but why? Why can't I just bleed the front brakes until all the air is gone? Won't it keep pushing the air through? Does some air stay in the system no matter what until it is bench bled out?

Obviously there must a reason for bench bleeding. I just don't understand how pushing air through a bench bleed kit is different than pushing it all the way out to the caliper's bleeder screw.

Easier to properly bleed the brakes if the master has already been bled, not required but helpfull. I've done many brake jobs and bled them without bench bleeding the master first. Your call.

By bleeding the master on the bench you are guaranteed to push the piston through its FULL stroke, getting all the air out of the chambers/valving, whereas "on the car" you'll only be able to pump it a percentage of possible travel.

A bench bleeding kit is available at most good parts outfits for about $10. I do it on all master cyl. jobs.

My .02

I understand the primary purpose of bench bleeding the master is to get out that air that will be trapped in the top of the master cylinder chamber when it is on the car and sitting at an angle. Remember that the master is tilted quite high at the front when installed. Bubbles can sit in the high corner and not be pushed out the exit hole or bleed screw which is lower than the top corner. For this same reason, calipers must be positioned so that the bleed screw is at the top of the cavity with fluid. If it is in the middle or bottom you will never get the air out.

I have a related question... if i have basically an empty rear reservoir, but the front is full, can i just add fluid to the rear reservoir and bleed all of the air out of the rear brakes? i really don't want to crack open the lines on the master cylinder if i don't need to. I guess my question is, is it possible to get all the air out bleeding them at the brakes, or will there be a large "pocket" of air up near the master cylinder?

If the rear chamber hasn't gone dry you should be OK. Just top it off and bleed away. Keep an eye on the level while doing it ;)

I agree with JohnD. Top up and try to bleed all the air out of the rear. Should work. It is the front chamber that is drastically tilted up where air can hide in a top corner.

post stealing time
how do u bench bleed. what do u push in to power booster or mc to bench bleed. the kit i got with mccp was just some clear rubber lines. im lost. and i want to do this 2marrow so i can drive it fri. after i do the brakes ill be ready to run assuming i get done with the wiring behind the booster at the same time.
ps
remember a picture is worth a thousand words. ;) pics helpful

Put your master cylinder in a vise. Make sure it sit's level. Attach the clear tubes to each port on master. Loop other end of each clear tube (opposite of end attached to port) into the resevoir above it. Fill both sides of master with brake fluid.Make sure both ends of clear tubes are submerged in the fluid. Use your pedal rod or phillips screwdriver to slowly push in the master piston. You will see the air bubbles coming out into the fluid. Keep doing it for several minutes until no more bubbles appear, and you'll be good to go.Hope that helps . Good luck Don :cool: