Brake Booster Check Valve Question

[240260280z]

Also consider lack of free play in the master brake cylinder or and incorrect master or a malfunction in the master where it does not recoil.

[Captain Obvious]

Gotcha on the check valve.

 

So the brakes worked fine for some time before they started to tighten up? That's odd... Makes me think there might be something wrong with the check valve in the master cylinder. Are you sure it's both front corners and not just one side?

 

I'm no expert, but I wouldn't expect it to be a mechanical issue with the pad pins. Problems with those could cause a little drag, but I don't think they would cause enough of a problem that you would notice it in the car being dragged down. I would also consider it unlikely that both rubber hoses collapsed inside at the same time. They do go bad, but both sides at the exact same time?

 

Master cylinder check valve? Rebuilt caliper(s) that sat on the shelf so long that the pistons corroded up again?

 

Good luck with the investigation.

[Zed Head]

Just for fun, and a reason to study one of these excellent Nissan drawings.  As I understand things, it's possible to adjust the MC rod so far forward that the return hole to the reservoir, which also releases pressure, is never opened.  Brakes are bled cold, fluid heats up and expands, the pads and shoes are moved, car is slowed.

 

I highlighted the two holes that can get covered.  Never had it happen to me but it seems feasible and has been reported.  You can see how the piston seals are right next to the holes, so that pressure is created as soon as the pistons move.

[Hardway]

Good news! I got my brake issue figured out.  I started to jack up the front of the car and when I did I took a second to try and spin the wheels.  Sure enough, they spun with no hesitation.  I let it back down and jacked up the rear of the car, passenger side drum moved with just the right amount of resistance, the driver side was locked up.  Got the wheel off and used a pry bar to turn the drum and hub.  Once I could get to the adjuster I loosened it and got the drum off without issue.  Everything looked good inside, the adjuster was not chewed up so I put it back together and tightened it up to match the other side.  I put it back on the ground, reinstalled the check valve and hoses.  I took it out for my same 5 mile test drive and all the brake components seem to be working properly.  I am guessing that over time the parking brake mechanism might have ratcheted the left rear brake shoe too tight.  I will keep an eye on it and if it happens again I may look at replacing the wheel cylinder or adjuster if I can buy one separately. 

 

As always, thank you to everyone who contributed to this thread.  I am sure it will help other Z owners in the future.

[Captain Obvious]

You know... I was gonna ask. It's my job after all!  I should have asked. I should have asked.

 

Glad you got the initial issue worked out, and I hope it stays worked out. I'm really unsure though how such a thing could have happened in the first place. The adjuster mechanism is pretty simple and it should be impossible for it to over tighten.

 

I've never tried, so I don't know if it's even possible, but what are the chances that you got parts mixed up from one side to the other and the brakes are auto-adjusting when they shouldn't be? You've got the older floating cylinder design, right?

[Hardway]

Good question Captain Obvious.  Yes, these have the floating rear wheel cylinders, held in place by two opposing clips.  I thought about your question about the rear wheel cylinder possibly being on the wrong side and I do not think it can be done.  The main reason for this is if you had the wheel cylinder on the wrong side the hard line would not line up and the parking brake cable would not work properly. 

[Captain Obvious]

Yeah, I'm just tossing out ideas. Not necessarily GOOD ideas...      I can't really come up with anything definitive that would cause overadjusting of the rear brake.

 

So what happens if the cylinder doesn't slide properly on the backing plate or if it's not centered properly between the two when you put the drum on? I know the whole thing is supposed to center properly the first time you hit the brakes, but what if... What if it doesn't?

 

Is it possible that the piston extends and pushes the one shoe out too far against the drum. And is it also possible that the shoe itself pushes against the parking brake lever causing the ratchet arm to give the wheel a notch or two? Hit your brakes fifty times and you've got fifty teeth?

 

I don't like that floating cylinder design and I'm just glad that my 77 doesn't have that style. I had enough of the early style with my 74.

 

Maybe just keep an eye on it and hopefully it just won't happen again?

[Zed Head]

I think that it might be possible for the self-adjusting mechanism to over-turn the wheel in the adjuster.  I've studied it and the little arm that grabs a tooth and moves it is meant to have a certain limited travel.  So that it can only grab the next tooth after certain amount of wear allows the lever's starting point to drop back to the next tooth.  And the width of the arm causes it to ride on top of the teeth until the proper moment arrives.  For many brake lever pulls the adjusting arm just slides back and forth on top of the adjusting wheel teeth.  Until, finally, it can drop down, grab a tooth, and move it forward.

 

Considering the beating that brakes take when we replace the shoes it wouldn't be a surprise to find that arm malformed or worn.

 

Could also be that the external stop, or bumper, that determines the starting position of the external portion of the brake lever,  is allowing it to drop back too far, allowing it to grab a tooth early and move it too far.