Bleeding Brakes / Brake Master Cylinder

[ea6driver]

78 280Z

Background....about a year ago I completely disassembled everything in my engine bay so I could repaint the bay, refurbish all the engine compartment components and then reinstall it all.  I'm finished up with it all now...the last remaining task before getting the Z back on the road is bleeding all four brakes.

 

 

 

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So today I jacked up the rear, removed the wheels and started the process by trying to bleed the left rear brake.

 

 

 

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As an aside, before I started this "refurbish" process, the brakes were working perfectly...very firm, even braking from all four brakes.

Using a Motive Products Power Bleeder, pressurized to 15 psi, I was unable to get a steady flow of brake fluid from either rear brake bleeder port.  Additionally, when I tried pushing the brake pedal it was very firm...as in I could barely push it down at all.

Next I connected clear tubes to both of the master cylinder bleeder ports an stuck the other end of the tubes into the reservoir (see photo).  I opened the bleeder screws and was easily able to pump the brake pedal, circulating brake fluid from the bleeder into the reservoirs.

So, the question is, why is the brake pedal so firm with the master cylinder bleeder screws closed, but not with the bleeder screws open?

I'm thinking 1) the brake master is malfunctioning 2) there is some kind of blockage preventing brake fluid from flowing freely from the master cylinder to the rear brakes...perhaps a problem with the NP valve (proportioning valve).

Eager to hear your ideas.

 

[ea6driver]

One day later....

So I spent the afternoon trying to figure out this problem.  After checking all the basic connections, I finally decided to start disassembling the components to check them out individually.  The first component to come out was the brake master cylinder.  Upon disconnecting the two brake lines from the bottom of the master cylinder, I immediately noticed that the brake lines were blocked/plugged up with the remnants of paper towel.

Apparently, in my haste to reassemble everything I forgot to remove all of the paper towel pieces that I had used to plug up the lines/ports while everything was apart.  After a few hours of blowing out lines with compressed air and taking apart the master cylinder I'm pretty sure that I've now successfully removed all the offending material.

I feel like Homer Simpson....DOH!!!! A self imposed headache for sure. My wife said that I shouldn't confess to this mistake on the forum...but I felt obligated to disclose what the solution was to the problem that I originally posted...maybe it will help someone else out sometime down the line...

I'm feeling confident that tomorrow the brake bleeding job will go much easier!

[Zed Head]

I read your first post and wanted to reply but didn't have a clue.  Those rags and paper towels are common problem sources, form what I've seen.  You should keep a count of your paper towels like a surgical team with sponges, and verify all accounted for after a procedure.

[dutchzcarguy]

On 10/1/2020 at 6:33 AM, ea6driver said:

So, the question is, why is the brake pedal so firm with the master cylinder bleeder screws closed, but not with the bleeder screws open?

Sorry but if that is not clear to you, you better leave that brake job to a qualified mechanic.. oeps..

[grannyknot]

8 hours ago, ea6driver said:

I feel like Homer Simpson....DOH!!!! A self imposed headache for sure. My wife said that I shouldn't confess to this mistake on the forum...but I felt obligated to disclose what the solution was to the problem that I originally posted...maybe it will help someone else out sometime down the line...

We all do stuff like that, some admit it, some don't.

[Pilgrim]

Admitting a screw- up is a nice way to help someone else avoid it.  All of us have brain farts on occasion.