Brake Bleeding Problem
 

I was getting a soft brake pedal and decided to subsequently check my MC. I would compress the pedal about 1/8 to 1/4 of the travel and it would firm up, but if I kept pressure on the pedal it would move maybe 1/4 to 1/2cm per second downward, but mind you under a firm foot. I decided that the MC must be exhibiting symptoms of failure.

So, new ATE MC in the mail. Installed. BTW, during the install a good amount of gook in the reservoir - cleaned that out well. Used a Motive Power Bleeder to bleed the brakes. Flowed well. I did not bench bleed per advice on not being needed with using Motive. The technique used to bleed was to open the bleeder valve at the wheel cylinder, have my lovely helper slowly pump the brakes 12x as I left the valve open, holding on the last pump, per Bentley manual.

Buttoned it up. Firm pedal when sitting in garage. Start the car, roll out to the drive way, and it is really soft in the first 1/4 of the pedal travel, then spongy, then firm but feels like it is not holding firm, just like the initial issue.

So, I decided I would bleed again... :rolleyes:

This time, bleed again with the Motive, but did 10x pumps and hold BEFORE opening valve. This time, it seemed to really firm up the pedal. Would flow clear without bubbles for 2-3 minutes (~300ml) per wheel, but then it would start flowing bubbles and never seemed to stop until I filled the rest of the Motive catch bottle (about the size of a sports bottle). This was the same for each wheel... bubbles just wouldn't stop (small to medium bubbles, not big ones). Thought, well, I have bled 2 full liters of fluid and still bubbling, so I will just wrap it up and test drive again.

Again, firm high pedal in garage after completion... until I start the motor. Soft for the first 1/4 travel, then spongy, then ever so slowly sinks (never to the floor, but would travel down about 1/3 of the travel from the top in 10-20 seconds of pressure).

Could not find any leaks at the wheels or under the new MC.

Thoughts?

I would check the caliper piston,make sure they freely and not rusty inside behind the rubber booth.

I use a check valve attached to the bleed line with a bottle. Prevents accidentally sucking air back in. Never had a problem this way. I have a Motive and it sits on the shelf collecting dust. It seems like more hassle than necessary now.

It does sound like you are still getting air in somewhere. I like the idea above of checking the caliper pistons and seals.

Need to depress the pedal, open bleed screw, close screw and repeat. air will often get in through the screw threads. The "one man bleeder" helps in that it will not allow air back into the system. Personally I use a vacuum bleed system at the caliper.

Chris

I use a vacuum bleed set up also. Another thing I do is to put just a little bit of grease on the bleeder screw threads so air can't get sucked back in through them.

Craig

Some say the Motive is fool proof and yet there have been multiple threads with them. What's the scoop ?

http://forums.pelicanparts.com/porsche-911-technical-forum/1048603-problem-bleeding-brakes.html#post10704022

http://forums.pelicanparts.com/porsche-911-technical-forum/1039028-bleeding-brakes.html

I use Motive pressure at the MC and vacuum at the calipers. More is better, right?

I have used a variety of methods bleeding various calipers on lots of cars and trucks, including about over half dozen different Porsches and BMWs.

Most typically I use a Mityvac to pull fluid through... but I also use an assistant to do hand bleeding from time to time.

With the Mityvac, I try to close the bleed screws under vacuum so no air can get back in the caliper by passing along the bleed screw threads.

When I had bleed, my assistant is not permitted to pump too fast or bottom out the pedal. And I close the bleed screws under pressure.

I also have and use a Motive tool from time to time. Again, with this tool I close the bleed screws under pressure.

The Motive tool works great. I know the owners of the company and they are car guys just like us.

And by the way, I have never been successful using the existence or lack of visible bubbles in clear tubing as any kind of indicator. My indicator is a firm pedal when I am done.

My Bleedin' Speed Bleeders work well Introduction - Speed Bleeders - Russell Performance Products

Have speed bleeders on my SC.

There's also the vacuum bleeder method using the hand pump. Raw's apparently very well versed with that technique.

False! The suck method is only applicable to non-turbo 911s, bro!

Yeah, yeah, blowing's better than sucking as far as you turbo boyz are concerned.

But ... in this brake bleeding context, technically which is superior ?

Nutz, pressure applied to either the master cylinder res (with or without a remote fluid source) or the caliper bleed nips :eek: (reverse bleeding - very good, but requires a remote fluid source) is far superior to vacuum bleeding (even if a pneumatic vac bleeder device is used) - the vacuum method tends to draw fluid quickly while moving trapped air very slowly and the pressure method does not (despite what you might sometimes read here in the tech forums). There is nothing better than pressure bleeding (using an air compressor - fook that pump bottle stuff!), in terms of speed and ease - gravity and pedal pumping are 912 caveman methods in comparison.

But then we have this thread and others having problems with their "blowing" techniques with no apparent resolution so it doesn't appear foolproof. Is it the Motive which is the problem ? The vacuum method with a hand pump (which most have lying round, not no that type, in the garage) at low psi can't be that quick in drawing the fluid.

I've never used a Motive hand operated pressure device of any kind (compressor only for top down bleeding and a purpose built fluid & pressure tank (air pressure supplied to it via a compressor) for bottom up bleeding, so I can't comment on whether or not there is a problem with its design. More than likely, the issues that some people run into with pressure bleeding are caused by improper technique. I have use pressure to bleed all manner of brakes (aircraft, caveman cars, newer cars with mechanical-style anti-lock brake systems, big ass truck with seemingly hundreds of feet of brake lines, hoses and anti-lock cylinders, etc.) and never once have had an issue (including starting from totally dry when most of the brake components were replaced). And I've never had to bench bleed a master or clutch slave cylinder when using pressure.

By "quickly", regarding the vac method, I just meant quicker than gravity or pressure pump methods (but you knew that, ya' bisch!!! :D).

Wow, lotta sucking and blowin, but hydraulics were invented by man and must eventuall
 

Lot of experience been voiced , I’m thinking you have air entry somewhere, reservoir feed hose to master cylinder, bad mc, as suggested, leaky piston seals, or bleeder screws, or less likely, leaky flex hoses / steel lines.
At this point, I would suggest doing a carefully progressive, visual inspection for leaks. Have a suitable assistant put constant pressure on the pedal, while you look for leaks, starting at the mc feed from the reservoir, then downstream.
On the pressure side, I’m having trouble believing that it would suck air, but not blow fluid under foot pressure. Ie, the bLed screw or piston
Despite all the excited sucking and blowing in some of the prior posts, you will find the leak, air entry.
I am kind of leaning toward a bad reservoir nipple, feed line or sucky new MC.
Cheers,
chris