M/C - bench bleed or not?
 

I'm about to replace the M/C on my 912E barn find.
I've talked to several 'master' mechanics and a bunch of shade tree guys.
No definitive answer that I can discern here.
I'm oh-for-four for M/C replacements on my daily drivers, so I'd like to get this one right the first time.

Even after you bench bleed, can't air still get into the unit when you transfer the unit to the install location?

FWIW, the 912E is unboosted, and non-power - location is under the car above the splash guard.

Thanks in advance,
Bill K

Save yourself some time and do a bench bleed.

Why?
And how will it save time?
Bill K

I just pressure bleed after install. Never had any problems.

If you bench bleed, you'll get out most of the air. You'll introduce some air when you connect up the lines, but that will be fairly easy to get out of the bleed fittings. The less air you have to bleed out of the calipers, the faster this will go.

I'm sure John is right that pressure bleeding works fine, but if you don't have that system, I think bench bleeding is worthwhile.

Mark

I didn't and just used my Motive. Maybe took awhile to fully bleed but I have nothing to compare against. Just know it worked. Having fluid in the MC while installing has got to increase the mess factor of an already messy job you'd think. Trying to keep stray or dripping brake fluid out of the frunk is more useful to me.

If you are a home auto repair guy a bench bleed will save you some aggravation. If you have a good pressure bleeding solution it won't make a huge difference. (But if you had a good pressure bleeding system you wouldn't be asking would you!)

outside of the mess, bench bleeding isn't going to hurt. that being said, I couldn't go back to life before the motive power bleeder.

just top off the reservoir and put 5psi through the motive. the whole car is done in 15min and you don't have to ask your wife to pump the brake pedal.

ouch !!

fwiw, I do have a Motive, and a Mighty Vac.
Bill K

Not sure what the "ouch" is about. :confused:

I like the Motive if you run it dry to just add pressure but it needs a cutoff valve so you can remove it to add fluid without blowing off all the pressure. If I was setting up a pressure system again I would just get a cheap Harbor Freight pressure regulator and attach it to a spare cap. Would save money, storage space, and time.

Not a big fan of the Mighty Vac or other vacuum systems. The seals in a brake system are designed to seal under pressure. A vacuum reduces the seal's effectiveness and it can pull air past the seals creating a "Why the hell won't it bleed?" situation that will drive you crazy. It isn't a likely outcome but if it happens to you once it will sour you on it forever.

Just untwist the large cap on the top of the Motive. As soon as it gets even the slightest bit loose, it releases pressure at the bottle instead of at the brake reservoir.

I run my motive dry. Drilled a hole through the top and installed a metal tire valve. Just depress the shrader valve to release pressure.

Bench bleeding helps get the siphon action going when manually bleeding. When pressure bleeding; this is not an issue (no bench bleed needed).

I recently used a Motive unit for the first time on my '84. I will never do it any other way. It's one of those tools I should have bought years ago.

And I wouldn't bench bleed. Cleaner and easier to just use a little more brake fluid with the Motive.

My experience with the Motive has been excellent on a number of cars and I have avoided bench bleeding the master. I tend to release the pressure by opening the motive container after each corner, top up the reservoir and re-pressurize. Just did my 911 after replacing the flexible lines last week, and instant pedal.

Replacing a master cylinder in that location is a PITA. Bleeding bubbles out will be the least of your worries. :)

if the MC is good then it should be easy to bleed relying on gravity alone even. Just fill reservoir and open one bleed nipple (with transparent hose on nipple) at the time until clean fluid. Drive car slowly around the block and repeat. I only had problems once and believe I had a bad MC then

I like this post.

I never did a 911 MC but lots of American cars. I always bench bleed.

However, if the space is confined on the 911 and connecting the hard lines will be a PITA - not something that takes seconds - I like the idea of installing first - bleeding later.

Also, with American cars that folks have run empty and fill the MC with air, you can crack the lines at the MC and bleed lots of air out with a friend. Bummer is on a 911, fluid does not simply fall to the ground.