nstrug
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| posted on 29/8/09 at 03:21 PM |
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Brakes locking - Again!!
I know this subject has been done to death but I'm at my wit's end. My rear brakes hydraulic locked again today. Unfortunately I
didn't notice until I saw the huge plume of smoke coming out the back of the car. The pads were completely burnt and the caliper paint is
bubbled.
There was still free play in the pedal but it was clearly hydraulic lock as the brakes released as soon as I undid the M/C brake union.
Now this is a brand new 0.7in Girling M/C on a bias bar setup. There is plenty of free play - in fact it's a completely floating overhung pedal
with no pedal stops.
So I'm at a loss as to why I'm getting hydraulic lock. I never got this with the old 0.625in M/C so possibly the new one is faulty?
I would really prefer to use a 0.7in M/C on the rear circuit as it give much better pedal feel.
I'm going to swap back in the 0.625in tomorrow, but I would really like to know why this might be happening. If anyone nearby (West Surrey)
knows anything about brakes, they're welcome to come and try and figure it out - beers waiting...
Nick
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nstrug
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| posted on 29/8/09 at 03:32 PM |
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I should add that I don't have any additional return springs on the brake pedal - would this help? There seems to be plenty of free play so I
assumed it wouldn't be needed.
Nick
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David Jenkins
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| posted on 29/8/09 at 03:39 PM |
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The pistons in the master cylinders have to come right back to the off position to allow excess fluid back into the reservoir - are you sure
it's coming back to the fully-off position?
And yes - a big strong spring is very useful to make sure! On my car the spring is hefty - you won't stretch it by hand. Leg muscles and a lot
of leverage can stretch it with no problems though.
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nstrug
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| posted on 29/8/09 at 03:47 PM |
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quote:
Originally posted by David Jenkins
The pistons in the master cylinders have to come right back to the off position to allow excess fluid back into the reservoir - are you sure
it's coming back to the fully-off position?
And yes - a big strong spring is very useful to make sure! On my car the spring is hefty - you won't stretch it by hand. Leg muscles and a lot
of leverage can stretch it with no problems though.
The rear M/C piston certainly looks like it's coming all the way back but I'm thinking that as the pedal isn't hanging vertical
maybe the weight of the pedal is just enough to keep the valve shut?
In which case fitting a hefty spring should do the trick right?
I tried hooking the pedal up with my toe, but it didn't release it at all - had to crack the union - great big jet of fluid so obviously under
quite a lot of pressure.
I'll try the spring tomorrow but I'm really nervous driving the car now - especially after the flames...
Nick
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britishtrident
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| posted on 29/8/09 at 03:52 PM |
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The problem is likely to be the washer held in by a circlip at the end of the cylinder not allowing the mushroomed end of the push rod to come fully
back. Even if it stops it by less than 1mm it will cause hydraulic lock.
Get the master cylinder out on the bench and try a bit of hand fettling on the push rod and washer to gain more movement.
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MikeRJ
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| posted on 29/8/09 at 08:48 PM |
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quote:
Originally posted by britishtrident
Get the master cylinder out on the bench and try a bit of hand fettling on the push rod and washer to gain more movement.
I had to do this on the Striker M/C, I simply made the washer a deeper "dish" shape to accommodate the mushroom on the end of the push
rod.
[Edited on 29/8/09 by MikeRJ]
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nstrug
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| posted on 29/8/09 at 10:36 PM |
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Don't think that Girling M/Cs have a washer, there's just a circlip that holds the pushrod mushroom in (at least according to the
diagrams). It might be possible to move the circlip back slightly in the bore so I'll take a look.
Nick
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nstrug
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| posted on 30/8/09 at 10:42 AM |
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Ok, so I stripped down the 0.7in m/c and also the old 0.625in m/c for comparison. Here's what I discovered:
1. The retaining washer (I was wrong, it does have one) on the 0.7 m/c was undersized and loose, possibly causing interference.
2. There was NO grease on the mushroom, so it was just bare metal on metal between the mushroom and piston. The 0.625in by comparison was full of red
moly.
3. Worst of all....
The 0.7in M/C WAS FULL OF ALUMINIUM SWARF!!!!  
Looking at the casting, I don't think this is a real Girling at all - probably a crappy chinese knock-off.
The supplier will be getting a phone call on Tuesday morning....
Nick
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nstrug
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| posted on 30/8/09 at 10:51 AM |
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Oh, it gets better, the shroud round the valve seal is actually shorter than the valve seat so there is zero standoff. This means that the _slightest_
amount of pressure on the piston will cause the valve to close.
On the real Girling the shroud has about 1mm standoff so the piston must move at least 1mm before the valve shuts.
This thing is a deathtrap
Nick
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