# Brake problem (they won't turn off!)



## tscman (Jun 23, 2005)

Hi guys,

Sorry this is a repost; originally posted in "Classic Datsuns"..

I don't know if anyone on this board still works with the series II bluebirds of circa '84, but if you happen to know anything about them and could help me, I'd be very grateful as I am pretty much at my witt's end with this little problem...

Has anyone encountered anything like this before?

I have an '84 bluebird auto, with the L20B, everything stock standard. The problem I am having with it at the moment is that the brakes are constantly on.

Very annoying - it's effects on fuel consumption go without saying...

To be more specific, when the car has been parked and everyhing is cold, the brakes are off, and the brake pedal works a couple of inches off the floor, where it normally does (brakes work as normal). After driving a while, the brake circuit seems to build up a constant hydraulic pressure which keeps the front brakes on constantly, and this becomes gradually worse and worse, the further you drive to the point where I can't get over 50 k's without the accelerator on the floor. And the brake pedal starts working right up the top (the feel of it changes).

The thing that completely baffles me is that I have replaced everything in the front brake circuit -

- replaced master brake cylinder (twice)
- replaced both callipers
- replaced brake pads (several times )
- replaced both flexible brake lines
- even replaced both steel brake lines !
- oh, checked obvious things like the vent in the master cylinder reservoir
- bled litres and litres of fluid

Rear brake circuit doesn't leak, and they don't get too hot. Front brakes absolutely cook after a long drive.

I have re-packed front wheel bearings couple of times with htb grease and checked adjustment to make sure they're surviving this heat...

If anyone has a suggestion which may save this car's life I welcome your post!

Thanks for the lengthy read,

David.


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## Matt93SE (Sep 17, 2003)

Damn.. I wish I could remember what we did on that Z31 a while back..

had the exact same problem with that car too.. new everything and it was still doing it.
wound up being a valve or springs or something.

Was this doing the same thing before you replaced the master cylinder?
I've found it hit-and-miss when buying parts store MCs.. I bought 5 of them in one week for 3 cars. (two of them were bad straight from the parts store!)
also, you did bench-bleed the new MC before installation, no?

just checking the obvious first.. good luck.


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## bahearn (Jul 15, 2002)

My first thought was master cylinder. Is the proportioning valve integral with the master cylinder? If not, have you replaced that yet? If so, I'd try another master cylinder.


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## MShorten (Jul 19, 2004)

OK, so you've eliminated the MC and everything else, so you've got two things to look at: 1) the power booster (if your car has one) - if it's failing due to getting hot, it could be applying braking pressure to your MC which would result in what you are talking about - perhaps there's a vacuum leak of some sort? 2) if the proportioning valve is not part of your MC, then I would change it. Sounds like it could be locking up somehow.

Good luck and let us know what you find?
Regards,
Michael


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## tscman (Jun 23, 2005)

Thanks for your many and quick replys, guys! Really awesome...

Matt93SE - yes I did bench bleed, thanks for the reminder - the first time I replaced the master cylinder I couldn't get a very hard pedal after sooo much bleeding. Then I came accross bench bleeding somewhere in the manual I think and that made the difference.

Hmm, it has a proportioning valve as a block of metal mounted on the rear axle; the rear brake line passes through it... but wouldn't this only affect hydraulic pressure in the rear brake circuit? (the bluebird is dual-circuit by the way)

Aha! the booster! Yeah, maybe that could somehow be holding the pushrod in... 'cause when I work the pedal I can feel the pushrod staying in... but I can feel that even when the engine is not running.

Thanks for your suggestions everyone; I will investigate the power booster and let you know


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## mklotz70 (Apr 23, 2005)

I'll throw in my 2 cents.....

Since the prob seemed to appear when you changed the m/c, it could be that the cup in the new ones are slightly different than the original. I'd check the linkage from the pedal and make sure that there is a small amount of play before it's engaged. If it's tight, then it may not be letting the m/c or the booster fully return. Since you have a booster, I suppose you should really check the linkage adj between the booster and the m/c. 


Is the return spring on the pedal?

All the datsun m/c's that I've seen in the books(and the ones I've had), all show residual valves where the line connects. I wonder if that's holding more pressure than it should be. You could try removing it and see if your prob goes away. In most braking systems, they're used on drums to keep the shoes close to the drums or where the m/c is below the level of the wheel cylinders....like the old under the floor board type on really old cars.

Master Power Brakes web site has a lot of really good info on how braking systems work....the new Street Rodder mag has an article by them too.


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## tscman (Jun 23, 2005)

*By Jove, I think I've got it!*

Thanks mklotz70. I can't find a residual valve... the manual makes no mention other than a "master cylinder compensating port" which can apparrently become blocked and is listed as a possible cause of my problem; that was the only thing I had left to check (but that would probably mean a rebuild of the master cylinder - or, with luck another mc - and I didn't want to go there since I'm on my 3rd mc). I will check the linkage; I had already loosened it off a bit...

But, I think you may have it with the power booster, MShorten!

And, if anyone else should ever have this sort of problem, this might save you heaps of time and trouble (although replacing brake system components at the sort of age my car is at is never really a bad thing )

I just backed the car up the drive way to roll it into the garage. As I parked it in front of the garage I noticed the creaking coming from the brakes, u know like u get in autos with it in drive and very light pressure on the brakes.

Anyway I though "crikey, they're on already and I haven't gone more than 10 metres!!" So whilst there was still pressure in the hydraulics and I could still hear this creaking (and there was enough brake resisitance that I couldn't push the car on the flat & level) - & with the engine OFF, I removed the master cylinder from the booster. The pushrod was sticking out a little, and slowly receeded back into its rest position as I removed the mc with a hissing sound. "Odd" I though. I started the engine, and the pushrod stuck out all the way, and stayed there for as long as it had vacuum after I turned the motor off.

Bingo! With the mc off the booster, there was no residual brake system pressure and I was able to push the car along now on the flat!

So simple! Kicking myself I didn't try this in the very first place, to think of all the trouble I had threading new steel lines through the danged engine bay etc. Still I'm glad I've replaced half the brake system which was probably needed anyway.

</short_novel>


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## Matt93SE (Sep 17, 2003)

That's some freaky stuff!

I wondered about the booster since it was the only thing you hadn't checked, but simply assumed your power assist would stop working if the booster failed. sounds like yours is exactly the opposite!
I'm glad you found the problem.


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## tscman (Jun 23, 2005)

Thanks Matt - yeah weird eh, there's a valve in the booster which lets atmospheric pressure into the pedal side of the diaphragm, and that is apparrently leaking so that the thing just turns on as soon as it gets a vaccuum.


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## MShorten (Jul 19, 2004)

tscman said:


> Thanks Matt - yeah weird eh, there's a valve in the booster which lets atmospheric pressure into the pedal side of the diaphragm, and that is apparrently leaking so that the thing just turns on as soon as it gets a vaccuum.


Very cool! I'm glad you figured it out, that's the most important thing. I'm going to stash that away in the "remember this if something wierd happens and you change the obvious" category.

How much does a replacement cost, I wonder? Let us know how it goes with the fix?

Regards,
Michael


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## bahearn (Jul 15, 2002)

MShorten said:


> ...I'm going to stash that away in the "remember this if something wierd happens and you change the obvious" category.


Same here. I wouldn't have expected the brake booster to cause this kind of problem when it failed.


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## tscman (Jun 23, 2005)

Well, a new booster was only $20 (Australian - about $15 US I guess) from the wreckers and I got to pull it myself. First thing I did before I installed it (after cleaning it up of course) was to plug it into the vacuum line and start the engine, and I noticed the output pushrod followed the input pushrod, as expected, instead of just shooting out to full travel when a vacuum was applied.

So, installed it, and so far the brakes are actually staying OFF (until i press the pedal!). :thumbup: 

Yay, hooray, perhaps I can say this is 'fixed'! Thanks for the suggestions!!!


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