For What Its Worth

Bleeding Brakes with a Vacuum pump

Bleeding the air out of the brake lines is often a problem.  There are a few tricks to it.  My suggestions.

When installing new flared lines I like to "swedge" them in or make sure there is a tight fit.   Tighten them once, loosen them and check that they seated all the way around and then tighten them back up.  look at the flare end closely and you should be able to see where it seated.  Be careful when you have a steel line going into an aluminum block.  The steel can scar the aluminum if over tightened.

Bleeders.  The valves that allow you to bleed the system.  If re-using old ones, make sure they are clean of dirt and freely let fluid flow out.  A small drill bit can be used, by hand, to chase the hole down the center to get the gunk out.  Some Teflon tape on the threads will help seal the valves so they don't pull as much air around the threads during the bleeding. 

Once all the lines are in and test fitted you are ready to put in the master cylinder with fluid.   I bench bleed the master cylinder.  There should be instructions with the master cylinder on how to do this.  Careful, brake fluid eats paint and there will be a few drips if bench bled.

If your system has a junction block with a brake light warning switch you need to take extra precautions.   The brake light switch is activated by a pressure differential valve.  It is easy to activate this switch during bleeding when front/rear pressure drops and the other is high.   The problem is this switch blocks the "low pressure" line to prevent fluid loss in an emergency.  It doesn't go back easily either.   See picture below.   It usually takes a small rod and the opposite system without pressure to get it to go back.  Problem is the brake system has to be opened up and you have to do bleeding again. 

The old time fix is to take out the switch, circled in red, and put in a brake tool that holds the valve in place. The tool used to be easy to find.  Today it may be harder to find, or you need to make one.

Note this picture shows a junction block with an internal pressure regulator for the rear brakes.  Some times the pressure regulator is external to the junction block, on the line to the rear brakes.

Check master cylinder fluid level often during the bleeding process.  If fluid level drops to 0, you get to start again.  Put the lid on to keep the dust out, but don't tighten.

Bleeding with a hand vacuum tool.  

Put a box end wrench on the bleeder and make sure you can open and close it without taking the wrench off.   Then connect the vacuum tool.  The hose should make an air tight fit on the end of the bleeder.   Make sure you have the catch cup  in the line for catching the fluid, pumping fluid through the pump is hard on it.   With the bleeder closed, pump down the vacuum.  This is the trick.  Don't try to bleed with low vacuum, get as much vacuum as you can then crack the bleeder loose.  More than 15 on the vacuum gauge, 20 is good.  (My altitude is 400 ft. above sea level)

When the vacuum gage drops to low vacuum, close the bleeder.   Pump down and repeat as needed.  When there is air, the vacuum drops quickly. When just fluid it drops slowly.  You may still get air pulled around the bleeder threads and see air bubbles in the bleeder line. This is not a problem unless the bleeder leaks badly.  When are you done?   I usually fill the small catch cup 1/2 to 3/4 full for each wheel.  It is more by experience than measure.  Don't try to re-use old fluid.  New brake fluid is cheap.

Start at the rear, passenger side, then rear driver side, front passenger and front driver.   You may have to do this a second time around when lines have been drained or replaced or just to satisfy the worry nerve.   The second time is a lot faster if needed.

Test pedal, but first make sure lid is on master cylinder tightly.   With disc brakes you may have to push a few times to set the pads.  If the pedal feels good push and maintain pressure for 30 seconds.  Now check for leaks at any connection you have opened or replaced.

Happy Motoring, VHubbard. Oct 2009 Updated Aug 2014

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