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Author Topic: Brake imbalance  (Read 1383 times)

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Greenbay packer

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Brake imbalance
« on: 24 June 2018, 12:17:15 »

Around this time last year I replaced the front brake discs and pads on my 1996 V6. Not long after that I failed an MOT with a front brake imbalance being one of the reasons for the failure - I wasn't surprised as I could feel a slight pull to the right when braking. The MOT tester said it looked like left side was working slightly slower than the right, which would explain the pull to the right if that side was braking first, and said it might be a caliper or brake hose issue. I wasn't aware of any imbalance before changing the discs and pads but can't guarantee there wasn't an issue developing. 

When I got home I checked everything, but couldn't find anything obviously wrong, so decided to work the caliper piston in and out a few times to see if that freed it if it was sticking a bit. I did that by putting in the worn pads back in so the piston would come out much further then pushing the piston back in with the bleed nipple open. At the same time I had a look under the boot over the piston and it was remarkably clean with no pitting or leaks from the seal. I also thought that rebleeding might help in case I'd missed an air bubble last time. I took the car back for a retest and it passed, but only just. I could still feel a slight pull but got the impression I might have achieved something with what I was doing.

I didn't do any more at the time as the imbalance only felt very slight and I hoped the system might bed in with use. Its now coming up to MOT time again and as I can still feel a slight use pull to the right I've had another look at the N/S front brake to see if any issue there has become more obvious. The disc still looks very good, the pads show even wear, the caliper piston still looks to be in very good clean condition and the sliders are still move freely.

As the imbalance showed up on the testers rolling road last year I'm discounting any suspension or tracking issue that might cause pulling to one side under breaking and I'm not convinced the brake hose is at fault as I would expect flow issues inside the hose to cause binding or slow brake release which is not happening. Everything keeps leading me back to the caliper being at fault and I'm wondering whether there is greater resistance on this side even though it appears to move out and push back in ok.

Many thanks if you've read through all this, I'd be grateful to hear whether others would agree that its most probably the caliper or whether there is something I've completely overlooked. I've thought about trying another caliper from a scrapper but its hard to guarantee it won't have been on the shelf for years and possibly seized and corroded.

Finally, does anyone know where I can get some bleed screws for the Lucas caliper - I'm not sure if they're a standard metric screw or if they are specific to that unit. Thank you.
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dave the builder

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Re: Brake imbalance
« Reply #1 on: 24 June 2018, 13:18:25 »

Do the pads move freely in the slides ,
had to file paint off brake pads before because they have been too tight  :-\

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terry paget

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Re: Brake imbalance
« Reply #2 on: 24 June 2018, 14:08:39 »

Are the pads all from the same set? Do the caliper sliders move freely? As it's a one piston caliper it depends upon the caliper moving freely in its guides. I had one rust solid.
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Entwood

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Re: Brake imbalance
« Reply #3 on: 24 June 2018, 14:45:28 »

Although the flexible brake hoses LOOK alright, its "possible" that, under pressure, one is "expanding" more than the other, so instead of moving the pads the hose expands first then moves the pad slightly later.

Might be worth checking the age of the flexible hoses and consider changing them ?? I doubt the slight expansion would be visible, its "possible" you might feel it if you hold the flexible pipe while someone brakes hard for you.

Remember, there is no "flow" in the brake lines at all, pressure applied at one end of the fluid is felt at the other as the fluid is incompressable, as the pipe expands the resultant pressure at the caliper is reduced, until the pipe stops expanding.
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Doctor Gollum

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Re: Brake imbalance
« Reply #4 on: 24 June 2018, 17:30:56 »

Also when was the brake fluid last changed?
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Greenbay packer

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Re: Brake imbalance
« Reply #5 on: 24 June 2018, 18:47:23 »

Do the pads move freely in the slides ,
had to file paint off brake pads before because they have been too tight  :-\

Pads move perfectly freely hence the even wear. I know what you mean about the paint though.
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Greenbay packer

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Re: Brake imbalance
« Reply #6 on: 24 June 2018, 18:49:43 »

Are the pads all from the same set? Do the caliper sliders move freely? As it's a one piston caliper it depends upon the caliper moving freely in its guides. I had one rust solid.

Pads are all from the same set, genuine GM, and the sliders are perfectly free. No sign of any corrosion on the piston as noted above.
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Greenbay packer

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Re: Brake imbalance
« Reply #7 on: 24 June 2018, 18:52:35 »

Also when was the brake fluid last changed?

12 months ago all round and yesterday for the NSF. 
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Greenbay packer

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Re: Brake imbalance
« Reply #8 on: 24 June 2018, 19:00:23 »

Although the flexible brake hoses LOOK alright, its "possible" that, under pressure, one is "expanding" more than the other, so instead of moving the pads the hose expands first then moves the pad slightly later.

Might be worth checking the age of the flexible hoses and consider changing them ?? I doubt the slight expansion would be visible, its "possible" you might feel it if you hold the flexible pipe while someone brakes hard for you.

Remember, there is no "flow" in the brake lines at all, pressure applied at one end of the fluid is felt at the other as the fluid is incompressable, as the pipe expands the resultant pressure at the caliper is reduced, until the pipe stops expanding.

Ok, maybe, I don't now how old the hoses are to be honest (5+ years since I've had the car)
but they look good and feel solid. I've not really considered expansion being an issue just thought about a constriction internally which often causes binding and slow release. Many, many years ago I had a hose on a Land Rover that went soft and squidgy and was probably expanding like you suggest but this one feels very firm and solid so I've thought it  was ok.
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terry paget

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Re: Brake imbalance
« Reply #9 on: 24 June 2018, 20:54:35 »

It could be the wishbone front bushes. I had a car, just passed its MOT, but before I handed it over to my son I wanted to correct a brake pull to the left. I had the wife drive it slowly over the inspection pit, with me in it, and brake. It then became obvious to me that the wishbone front bushes were unmatched, causing a steering twitch. I changed the wishbones, and the fault disappeared.
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Re: Brake imbalance
« Reply #10 on: 24 June 2018, 22:43:32 »

It's horrifying to watch an Omega brake hard from about 30 mph with ropey bushes... You wouldn't believe just how far the wheels move position if you haven't seen it :o
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Greenbay packer

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Re: Brake imbalance
« Reply #11 on: 25 June 2018, 08:46:32 »

It's horrifying to watch an Omega brake hard from about 30 mph with ropey bushes... You wouldn't believe just how far the wheels move position if you haven't seen it :o

I know what you mean as I had an Astra pull to the right some years ago and that was due to the bushes failing but would that show as a break imbalance on a MOT tester's rolling road? The test seemed to show that one brake was operating ahead of the other one. I will have another look at the bushes though.
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Greenbay packer

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Re: Brake imbalance
« Reply #12 on: 25 June 2018, 08:52:18 »

It could be the wishbone front bushes. I had a car, just passed its MOT, but before I handed it over to my son I wanted to correct a brake pull to the left. I had the wife drive it slowly over the inspection pit, with me in it, and brake. It then became obvious to me that the wishbone front bushes were unmatched, causing a steering twitch. I changed the wishbones, and the fault disappeared.

I'll have another look at the wishbones but was assuming this kind imbalance wouldn't show like it did on the MOT brake test, i.e., indicating that one brake was operating ahead of the other?
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Kevin Wood

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Re: Brake imbalance
« Reply #13 on: 25 June 2018, 09:18:40 »

Yep, if the MOT brake tester shows an imbalance then it's clearly not down to the bushes. That doesn't mean they are OK, of course. ;)

I can't remember how the brake circuits are split on an Omega. A problem anywhere other than the caliper would affect both sides equally if they're on the same circuit, which I think they are. :-\
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Greenbay packer

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Re: Brake imbalance
« Reply #14 on: 25 June 2018, 10:38:11 »

Yep, if the MOT brake tester shows an imbalance then it's clearly not down to the bushes. That doesn't mean they are OK, of course. ;)

I can't remember how the brake circuits are split on an Omega. A problem anywhere other than the caliper would affect both sides equally if they're on the same circuit, which I think they are. :-\

Thanks for that.  Its beginning to seem like I might as well start by looking at the caliper and either replace or overhaul it.
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