Bench Bleeding 15/16 280zx master cylinder

[zgeezer]

I am bleeding my master cylinder while installed in the car.  The power brake unit is from a 280zx 2+2 and is approximatly 10.5 inches in diameter.  I've run a clear plastic tubing  from each master cylinder bleed port to its corresponding fluid reservoir. I've filled each with DOT 5.

 

After two strokes pushing air, the front reservoir has a complete cycle: fluid enters the tubing and on the second stroke completes the circuit and re-enters the reservoir.

 

The back reservoir is different. On the first stroke fluid fills the tubing to almost the level of the loop back into the reservoir.  However, upon releasing the pedal, the fluid immediately returns to the master cylinder, leaving nothing in the tube.  On the next stroke, the fluid from the back reservoir will elevate in the tube; but, then immediately return to the master cylinder.

 

The brake pedal has a distinct feel to it:  It has a 1/4 inch of free travel before it engages the master cylinder.  Once engaged there is a smooth transition (with a tad of resistance) throughout the stroke until the very end.  At stroke's end there is a distinct "click" both felt and heard.  The pedal returns to the up position  easily and smoothly.

 

This master cylinder was purchased new several years ago and was filled, briefly, with DOT3 before it was removed, drained, and stored on a shelf in my storage unit.

 

I carefully measured the brake rod length on the bench with the vacuum unit out.   I "think" that dimension  is ok.

 

In terms of troubleshooting, what should be my next step?

 

I have another new 280zx ('83) that looks the same; but, the mounting ears are horizontal rather than vertical.  Is the later unit a "bolt one"?

 

Regards,

 

z

 

 

 

 

[Miles]

You are not "bench bleeding". 

 

You need to push the piston full stroke to eliminate air and pump enough fluid to reach the reservoir.

 

With the MC held in a vise on the "bench" you will be able to push the piston full stroke.

 

Go to the Brake forum and search for "bench bleeding" for the procedure.

[zgeezer]

You're right, of course, in that the mc is still on the car and not in my vice.  However, I'm looking for some direction as to why one side of the MC pumps and holds pressure, while the other does not.

 

I suppose my real question is rather or not these "symptoms" would point to a mis-adjusted rod or a faulty bleeding procedure or a faulty/corroded m/c.

 

I'm looking for direction.

 

z

 

 

[Miles]

You're right, of course, in that the mc is still on the car and not in my vice.  However, I'm looking for some direction as to why one side of the MC pumps and holds pressure, while the other does not.  Most likely trapped air.

 

I suppose my real question is rather or not these "symptoms" would point to a mis-adjusted rod or a faulty bleeding procedure or a faulty/corroded m/c. Since you are sure that the push rod length is ok, most likely the problem is that there is still air trapped in the MC.  Even small bubbles can be a problem. I would pull the MC and mount it in a vise so you can be sure to move the piston full stroke. When it is in the car the push rod is adjusted such that the piston can't bottom out which would eventually wear out the seals. Bench bleed then let it sit and the vise for awhile to allow the very small bubbles to collect together and pump the piston slowly. Repeat until there are zero small bubbles. Plug the outlets and move the MC to the car.  Attach the hard lines to the MC first and then push it onto the booster. It is easier to start the hard lines on the MC first  and avoid cross threading.

 

I'm looking for direction.

 

z

 

I'll see if I can post pictures or a link to bench bleeding etc.

 

http://forums.hybridz.org/topic/109115-master-cylinder-bench-bleeding/?hl=%2Bbench+%2Bbleeding

Edited May 5, 2016 by Miles

[NewZed]

looking for some direction as to why one side of the MC pumps and holds pressure, while the other does not.

I read your description as one cylinder (the MC is two cylinders stacked - in series basically) doesn't drain back and one does.  With an open bleed screw you're not really generating any pressure.  The back cylinder pressurizes the front cylinder.  Most of the FSM's have a really good drawing of the MC internals.  Some study there might give you a clue.  But...

 

Seems like you might be focused on an observation of something that doesn't really matter, along with the clicks at full stroke, which the MC will never see.  Is there an operational problem?