No fluid coming out at master cylinder bleeders - 1973 240Z

[bacarl]

Hoping to get some help on a problem that's got my brake bleeding buddy and me stumped. Bleeding the brakes is the last step in my suspension rebuild. The calipers have been rebuilt, the wheel cylinders have just been cleaned, and the rubber hoses were replaced with steel hoses. All pads and shoes are new but that shouldn't matter here.

We started with the right rear drum and weren't getting any fluid during normal bleeding so we moved up to the bleeders at the master cylinder and got no fluid there either. The reservoirs and filters had a lot of crud built up so, thinking there was a blockage, we removed and cleaned them, as well as the bores on top of the master cylinder with the reservoirs removed. Even now, no fluid comes through either the front or rear bleed screws, even with fluid in the reservoir and the bleed screws completely removed. In a last ditch effort, my buddy literally sucked fluid out of the bleed screw with a length of hose, and with a fair bit of pressure we could force fluid in and out of the MC, but it won't flow on its own, regardless of what you do at the pedal.

The brakes worked fine last summer when I started the suspension rebuild. I currently have fluid in the reservoirs and the screws open with hoses going down to a bottle to see if gravity pulls any fluid through over time but so far no dice, which is not surprising given the amount of force that was required to pull fluid out of the reservoirs. Any thoughts on what's going on here??

Seems like the issue must be inside the MC but things feel and sound like they're moving freely and there were no issues last year... help!

[EuroDat]

I have had a similar problem with another car, but could just as easy happen to a Zed.

The master cylinder builds up crud inthe cylinder bore over time. If you rebuild the brakes without doing the MC you can have problems withthe master cylinder. The piston get stuck when they got all the way to the bottom of travel and stay there. You don't notice because you are pumping brakes with a vacuum less booster. It just feels harder and different than normal. This could be your problem.

Its also not a good idea to run it with a "dirty" mc. It will probably fail soon after bleeding because the rubber cups are now most likely damaged by the pitting and grime in the bore section that was normally not wiped clean by the rubber cups.

Good luck
Chas

[bacarl]

Thanks for chiming in, Chas. We thought it might have something to do with the pistons stuck at the end of travel. If that's the case, are the piston(s) hanging up in some grime, and the springs can't push them back? Because the pedal moves freely and you can hear motion up in the MC so I think the rod that the pedal actuates is moving.

If they travel that far can they become permanently damaged by the pitting and grime in the bore? Or are you just staying that I should disassemble and clean, and I'll be in good shape?

[EuroDat]

The one we had trouble with came loose after tapping it with the handle of a screwdriver. You could hear it when it sprung back. It did it twice an then bled ok. We decided to order a new one, but we could bleed it for the time being. The new one came just in time. After two weeks the cups started getting so bad you could hold your foot on the peddle and it would slowly go to the floor.

When we changed it, we pulled it apart to see what was wrong. There was a lot of corrosion and pitting in the unused section of the bore and white aluminium oxide crud around the cups. The gug at the parts store could remember someone going througb the same problem, but he had to remove the mc and tap it on a block of wood to get the pistons out.

For as little as $50 you can get a new after market unit from rockauto. That is what I did and swapped my resivoirs the keep the original caps.

I think this is most likely your problem too. Maybe you can loosen the mc from the booster to look at the rear piston or even gently remove it from the booster and see if you can push them in with a screwdriver.

Chas

[bacarl]

Good info, thanks again! I'll try the screwdriver tap but I think I'll get a new MC on order to be prepared.

[bartsscooterservice]

Yes Chas is right, please put a new master brake cylinder, it isn't worth the risk...

If you start bleeding my advice is don't open the bleeders on the master cylinder, only with the front calipers and rear wheel cylinders. Fill up the reservoir and open up the front 2 bleeding screws first. Then gently pump ( use brake pedal ) the fluid into the lines and calipers, note:keep the reservoir filled, untill it comes out of the bleeding screws then close them, repait with the rear. Then when all beelding screws close you should allready have brake pressure, a little more bleeding is needed then, but that's the quickest way to get the air out, or of course you could get a vacuum bleeder..

[bacarl]

Just remembered this thread and wanted to follow up with my solution. Chas was correct, the pistons had stuck in the bore of the MC. They did not pop back when I tapped the MC with a screwdriver, but once I removed it from the car I banged the MC vertically on the floor a few times, and heard the pistons snap back to their nominal position.

It took a while to sort out some other issues with lines/leaks/bleeding, but the system is working now and feels pretty good.

Unfortunately, I failed to check my reaction disc when I had the MC off the booster... also I did not adjust the pushrod; I learned about these things after buttoning everything back up, of course

Currently, the pedal is quite soft and lacks feedback for maybe the first 30-40% of travel, then the brakes kick in and I get good feedback and stopping power. I think the brakes start dragging slightly during that first 40%, so it doesn't feel as though I'm using up free travel due to bad pushrod adjustment or a missing reaction disc. Can anyone with a stock brake system chime in on how the pedal feels through its travel?

[beermanpete]

When the reaction disk is missing the brakes feel unusually light and the pedal is low but otherwise seem ok at very light braking. When getting on the brakes more than a bit it gets grabby and unpredictable.

Correctly working brakes feel pretty much like any car with vacuum assisted brakes: Smooth, even, and easy to modulate. The pedal height should be about even with the throttle pedal (at idle) when braking is light to moderate.