Hey guys, SO I was driving my Z yesterday, and noticed i was losing power, then i realised I was stopping without using the brakes. I pulled into a parking lot and the front brakes were practically locked on, had a hard time moving in first.

at the same time, i noticed the brake pedal was very firm, and had almost no travel.

after letting it sit, the problem went away, and now i am getting front brake drag (but not as bad as before).

I am leaning towards calipers since this is a '90 and doubt they have ever been serviced. any other ideas?

 

sounds like a booster problem to me.

 

You could always take off a tire and inspect the caliper.

 

right, but im not really sure what to look for.

I was thinking of pulling the calipers and working the pistons and see if that frees em up.

 

are you sure it's only the front? it's unlikely that both front calipers will just lock up at any rate. you need to be looking farther up the lines either way. check the rears, don't drive it, and get back to us. if you drove it at all like that, be ready to replace the rotors and the pads.

 

It seems to be just the fronts, i went for a little drive, and the brakes got worse as time went on, even when not using them at all. I bled the rear brakes recently (replaced a bad line) but didnt do the fronts because the bleeders are toast. I am thinking there may be some water in the line, and as the brakes warm up, it is expanding until they are locked. cooling the brakes off helps to relieve it, but it keeps coming back.

Think I am going to try to get those bleeders out without breaking them, and hopefully it just needs some fresh fluid.

but it is definitely the fronts, both front rotors are very hot.

 

sounds like a good plan

 

bled the brakes, seemed to work, but same symptoms came back. im leaning towards master cylinder because the brake pedal is pretty hard.

 

a bad master cylinder just wouldn't do that. when a mc goes bad, it gets soft, because it allows fluid by the plunger.

but if the rear is really not doing anything weird, i guess that the booster is not the problem. did you bleed all the bleed points? there are more than four. you have to bleed from a component of the abs behind the passenger seat. i don't remember the details on that procedure though.

 

I didnt blead all the points, and i think it is weird that it is still happening. I didn't bleed the abs actuator because it didnt seem important. I guess I can do a full bleed, but it really doesnt seem like thats would cause this.

Another theory I have, is that the brake drag is too high and is heating up the rotor, which then expands and locks up.

how is the drag set on these brakes?

 

Just put it up on the lift to work on some other styuff, and noticed the rear brakes are getting tight like the fronts. There is just more grip on the fronts so i didnt notice with the wheels on the ground.

So it is effecting the whole system. master cylinder or booster?

 

booster man. i've never heard of a bad master cylinder causing anything but a soft peddle. considering front and rear brake lines are separate, i'd say you don't even have to think about the air issue anymore. you need to check out your booster and i can't get into any more detail on that issue.

 

all right, time for an update. I was playing with the brakes, and did things like make sure there was free play in the brake pedal (there previously wasnt), that didnt fix it. I then adjusted the booster output shaft length, which also seemed like it was too long, didnt work.

Front brakes locked up on my driveway after driving for a little while. I have snow on my driveway, so i just spun tires. I towed the car up the driveway with my truck, and all 4 tires were sliding on the snow. (out of gear and had brake was off)

So anyway, I disconnected the MC from the booster, and it didnt unlock, so it isnt the booster.

I cracked one of the bleeders on the abs actuator, and it relieved fluid pressure and the brakes unlocked. (all 4)

So I am stuck on either master cylinder, or ABS actuator.

any ideas? sorry for the long post.

 

Before replacing any component, drain *all* of your brake fluid and replace it (with high quality fluid). Brake fluid does go bad. And when it goes bad, its boiling point decreases. And then the fluid expands more. And expanding fluid locks the brakes. When you were opening bleed valves, you just released the pressure that was built up in the system from the expanded fluid. Once you drove it and used the brakes, the fluid heated up and expanded, causing the process to begin again.

***DO NOT FORGET TO BLEED IT PROPERLY, AS IN, FROM ALL BLEED POINTS***
If you do not bleed from the ABS actuator as well, you will not have properly functioning ABS.

I'm impressed at the responses provided, but equally depressed that nobody considered the less expensive solutions before jumping to the bigger ones.

 

except, that i already did a full bleed. and even if there is extra pressure, the pressure should be bleed back into the reservoir. when the brakes aren't actuated, there should be no residual pressure in the lines. The ports to the reservoir inside the MC should be open.

 

OMFG dude. A bleed is NOT fluid replacement. Did you not read a damn word I said about fluid going bad? Unless you remove the bad fluid, this problem will not go away.

Don't come here asking for solutions if you're not going to listen to them.

 

don't be a dick. I did a flush and bleed. fresh fluid out all bleeders

But that still doesnt change the fact that the lines should be at ATM (or close to it) if the brakes are released.

 

Don't call me a dick when you're the one rejecting the advice of the only person currently trying to help you. I could just as easily leave you to figure it out yourself.

When you step on the brake pedal, you are pressurizing the system greatly. If the fluid is bad, it will heat up and expand to the point that even when you release the pedal, the fluid will be so expanded that it is still pressurizing the system because the reservoir has a cap on it.

Now, if you've changed the fluid completely (which you had not previously mentioned at all and I honestly wonder if it's true, given that you doubt it as a cause), I would say your ABS unit is the likely culprit. The ABS unit's job is to lock and release the pressure in the system rapidly to prevent sliding... if the ABS unit isn't releasing the pressure properly, the brakes would stay locked.

However, the ABS unit is expensive to replace, so I'd worry about that last. Get a master cylinder rebuild kid and clean/rebuild the master cylinder. If the plunger isn't returning to the unloaded position, or if the ports are slightly clogged, it could be a culprit. I don't consider it likely, but a rebuild kit is cheap.

 

i understand the expansion. however, unless this MC is different, a standard MC opens the lines to the reservoir when the brakes are disengaged. Unless nissan put a check valve in the MC so fluid could not return back, it should equalize with the pressure in the reservoir.

and I understand how the ABS works, which is why I suggested it as a possible cause.

Although, when I relieved the pressure from the front ABS bleeder, it freed up the rears too. In the fsm, BR-25 there is a hydraulic diagram. The front and rear systems are not connected except for the circle thing between the 2 pumps. Do you know what that symbol is? I havent worked with hydraulic diagrams.

but anyway, If the 2 systems arent actually connected in the ABS actuator, then the only time the systems touch is in the MC.

edit: and when I take the cap off of the reservoir, it doesnt free it....

 

thought I would just summarize.

replaced the master cylinder and problem solved

 

interesting. thanks for the update.

 

So I know this is commenting on an old post but I'm having brake issues in the rear. They seize up when I'm not braking. I don't know if its the MC or the booster. Any help would be great

 

did you ever get your breaking issue solved? My breaks are locking up after a couple minutes of driving. Let it sit for a while and it’s good for a couple minutes.
Thanks for any advice.