Bleeding Rear Brakes on a '65 Forever

[carsngolf]

I changed the master cylinder on my '65 911 and it took seeming hours to get the air bubbles out of the rear calipers. The fronts were no problem but the rears would not "come clean". Has anyone else had this experience or is my incompetence showing again?

[old man neri]

How are you bleeding? Do you have a power bleeder?

[carsngolf]

Yes. I went through a bit over a quart of fluid.

[HarryD]

Reading your post, it looks like you started at the front (shortest lines) and then worked to the backs (longest lines). This has the effect of traps air bubbles all over the place.

You will need to repeat this but do the furthest one first, and work to the closest. The order should be right rear, left rear, right front, left front. If you did not prime the master cylinerm then you could have bubbles in ther as well.You will need to tap the master cylinder and brake calipers with a rubber hammer to dislodge air bubbles as well. You will go through 2-3 quarts this way.

An alternative is to "gravity bleed". This is sometimes the only way to get a dry brake system bled. To gravity bleed, you fill the brake reservior, and open all four bleeders. Put a tube on each bleeder to direct the flow through a "U". You then let the brake fluid flow by gravity in the system. Check the master cyliner every 5-10 minutes and top up as needed. When the bubbles stop, you are done (plan on 1-2 hours of fun).

[carsngolf]

Many thanks Harry. I would have sworn the proper technique was the shortest line first. I'll give it another try today.

[Fleming]

What is your technique for opening the valve?

With a pressure tank hooked to the reservoir: If you just open the valve to let a little out - the air bubbles just stay in the bends and corners - open/close the valve wide and fast to get the surge to dislodge the air pockets and bubbles.

2nd on the rubber hammer tapping on the caliper to move the bubbles through.

On the double valve rear calipers - outside first, inside second (recommended from an oldtime mechanic - could be a wives tale - but how I've done it since. Seems in line with the longest run first, shorter run after.)

hth.

[kodioneill]

longest lines first r/r l/r r/f l/f