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Old 09-11-2010, 09:33 AM
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"New" Bench Bleeding Procedure

I just replaced the M/C on my '80 Malibu wagon. The instruction that came w/it reads as follows:

ATTENTION
New and Improved Master Cylinder Bench Bleeding Procedure
  • Tighten bleeder plugs into ports.
  • Support M/C in a level position.
  • Fill half-way w/new fluid.
  • Install cover/cap.
  • Using blunt tool, (brake pedal if mounted to vehicle) SLOWLY press in and release the M/C piston using short strokes to 1. NEVER STROKE PISTON MORE THAN 1.
  • Repeat this step until resistance to piston movement is firm and less than 1/8.
I did it this way and had no problems except getting the plastic threaded plug on the larger of the two ports to seal.

The plugs are "stepped", having both size threads. This was OK on the smaller port, but the larger port allowed the plastic plug to bottom out against the hex head of the plug. The threads were short, so I wasn't too confident it would seal very good against the pressure. There was no leakage of fluid, but it bothered me just the same.

Most GM master cylinders use 9/16-18 threads for the front ports and 1/2-20 threads for the rear ports. You can make bolts work for this, just be careful to not damage the inverted flare seat in the ports. Wrap the threads real well w/teflon tape- this will tighten the fit of the bolt threads and help to seal the ports. Or use flared tubing and fittings w/the tubing soldered, brazed or welded shut.

What remains to be seen is, will this work on any/all other M/Cs.

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Old 09-11-2010, 11:51 AM
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Without the nippled fittings with hose attached and running back to the reservoir, where would the excess air in the cylinder go?

Maybe I'm not reading your post right, but you used a bolt in place of the nippled fitting?
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Old 09-11-2010, 12:39 PM
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Quote:
Originally Posted by sqzbox
Without the nippled fittings with hose attached and running back to the reservoir, where would the excess air in the cylinder go?

Maybe I'm not reading your post right, but you used a bolt in place of the nippled fitting?
I suggester that a bolt could be used- in my case, the M/C was supplied w/threaded plastic plugs to block the ports off with.

With the ports blocked, when the piston is depressed air bubbles out of the orifices in the bottom of the reservoir- first the front and w/a bit more travel, the rear.
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Old 09-11-2010, 02:25 PM
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I guess thats a new way to do it. As I see it, it's all about money.
With out the nippled fittings, two pieces of hose, and the clip to hold the hoses down in the reservoir, the kit would involve less parts.
I saved the accessory parts from one of the MC's a long time ago just in case I needed to re-bleed one or a used MC from the bone yard. The new procedure makes sense and I guess I could toss the bleed kit and make room in the tool box for other clutter!
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Old 09-11-2010, 05:19 PM
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This has been out here for a while, I have read some post with hoses going into the reservoir and other various MC bleeding tricks, never really understanding the procedure, figured if the guy has success then it's all good, the plastic plugs sure does make the MC bleeding a no brainier..

MP brakes supplies these with there MCs as well.
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Old 09-13-2010, 08:59 AM
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i always run the plastic tubes back into the master, just plugging the ports sure would make it a little easier.
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Old 09-13-2010, 09:12 AM
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Quote:
Originally Posted by THERACER
i always run the plastic tubes back into the master, just plugging the ports sure would make it a little easier.
Same here- that has always been the method I used, I didn't know there was another way except for pressure/vacuum bleeding.

I don't know if all MC's are OK to bench bleed this way, but I also hadn't swapped out a MC for quite some time- this is something that has come along in the interim for me. But it seems to have done the job.

I did get bubbles from the rear port that seemed to go on too long- like where is all this air coming from? But it may have been because the seals were new and only had the small amount of rust inhibitor in the cylinder to lube them. This may have allowed the seal to draw some air in reverse- but I cannot say that w/any certainty. It did seem excessive to me, though.
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Old 09-13-2010, 10:30 PM
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Does the "new to me also" method have something to do with the fact that most cars today have disc front and rear and there is a very low residule pressure valve or none at all in the MC ?
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Old 09-14-2010, 03:58 PM
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Quote:
Originally Posted by solidaxel
Does the "new to me also" method have something to do with the fact that most cars today have disc front and rear and there is a very low residule pressure valve or none at all in the MC ?
That IS a good theory- but the '80 Malibu this MC was for is a disc/drum set up.

It seems odd, though, that previously the bench bleed procedure was different and now so seemingly easy in comparison, w/o anything changing AFA how the MC is built or functions- at least AFA I know.

I would have thought that WE would've known about this before the rebuilders figured it out!
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Old 09-14-2010, 04:49 PM
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has anyone tried this on a M/C were used to fooling with
to confirm ?
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Old 09-14-2010, 05:28 PM
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This has been our standard procedure for all MCs for the last two years or so.

It works on everything we have tried it on. The most common MC we deal with is the early dual reservoir Corvette style. We have also used this procedure on some oddball stuff (like a '34 Olds, and '39 Olds, and several 30's Packards) and it seems to work equally well with all of them. We don't ever deal with late models (late model for us = 35 years old or newer), so I can't speak to the effectiveness on those.

Also, when bleeding the brakes, we put a block of wood under the pedal to keep it from ever bottoming out, and let the pedal up slowly rather than letting it spring back on it's own. Between that and the new bench bleeding procedure, we haven't had any problems with brake jobs in the last two years.

Knock on wood...


Quote:
Originally Posted by cobalt327
....I would have thought that WE would've known about this before the rebuilders figured it out!
My rebuilder, even after seeing it work many times, still refuses to accept it.
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