New MC on an SC: How/Why bench-bleed?
 

Along with my suspension work I am putting in a fresh MC to alleviate my concerns about rotting seals, and to take advantage of two new Porsche brake switches that are included in the MC package.

First question: I am told you must bench-bleed the MC - can anyone explain what this is and how it is done?

Second question: Do we have a Tech article on replacing the MC? Can anyone explain to me the procedure for doing this? For example, will I need to remove any of the pedal cluster, or simply the strut brace?

Many thanks for your help on this matter,
John

And we were just talking on another thread about how many people don't know that the home page has things like parts diagrams and tech articles. For example, here's one on rebuild a master cylinder along with great pics! Includes removal and installation. :D
911 Master Cylinder Replacement

Hi John,

There's an excellent Tech article here:

http://www.pelicanparts.com/techarticles/911_master_cylinder/911_master_cylinder.htm

I did mine last year and didn't bench bleed the MC, just pressure bled with a motive. It's an easy job, took me about 2 leisurely hours.

Cheers,

Tim.

Damn Kurt, you're just too quick for me!

My wife says the same thing.

I did not bench bleed mine, and did bench bleed my friends, both on SC's. Both were then pressure bled with a Motive pressure bleeder.

On mine it took a little longer to get all the air out, but was much cleaner.

It's my understanding that the bench bleed is unneccessary when you pressure bleed the new MC. I didn't bench bleed mine when I replaced it. Worked fine with the Motive bleeder.

Bench bleed is to help get air bubbles out of the MC body itself by force-feeding the MC with fluid from it's own pressure. You basically route the outputs back into the inlets to get the majority of the air out of the MC? I guess simply filling the reservoir and pedal pushing doesn't always accomplish this? That's my understanding (be it correct or not). As an aside, I had no issues when replacing the ATE MC on an Olds Aurora and not bench bleeding, nor pressure bleeding, the entire system. Only pedal pushing was done. Brakes work fine on it!

One thing to keep in mind. There's a torque spec. for those switches on the MC. Not very tight either. I remember screwing mine in pretty tight and then seeing the torque spec. sometime later, thinking, "oooh....... probably made those a bit too tight :( "

Thanks guys.

I believe the brake switches are a tapered fit so I pretty much put them in until I like the feel of the resistance.

John

Let us know how it goes john. Another of my future projects.

just installed all the lines to the MC. the tech article suggested putting the rubber grommets on the supply lines first and then sticking them into the MC. i tried for about 30 minutes with alot of cussing. i then popped the rubber into the mc and pushed the hardlines into the grommets. way easier! that tiny "click!" i heard as the metal lines slid home was just about the best noise i ever heard. i didnt prime the MC either, frankly i dont know how do do it without turning my garage into a superfund site.
cliff

The only purpose of bench bleeding is to remove the relatively large quantity of air in a new m/c before it is forced into the lines. Think of it as a short cut which avoids having more of an issue than necessary in ridding lines and calipers of air. You will also waste less fluid in the process.