extremely cr#p brakes

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extremely cr#p brakes

Postby Jin » Sat Aug 06, 2016 8:18 pm

Ever since I've had the car, the brakes have been rubbish, I'd sort of got used to it but I have had a few scary moments where I'm just not stopping quick enough.
The servo and master cylinder aren't that old having been replaced by the previous owner, the pedal isn't spongey and I've changed the fluid anyway as well as the pads and cleaned the disks (GTI setup).
The handbrake works a treat so I don't question the shoes or drums although I've not looked at them.
Any ideas?
Is there a very simple and cheap upgrade like a bolt on caliper with a bigger piston?

Any pointers much appreciated.
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Re: extremely cr#p brakes

Postby benofbrum » Sat Aug 06, 2016 8:39 pm

could it possibly be a partial obstruction in the hose between the manifold and servo?
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Re: extremely cr#p brakes

Postby Jin » Sun Aug 07, 2016 8:14 am

benofbrum wrote:could it possibly be a partial obstruction in the hose between the manifold and servo?



Hmm, not tried that, good point
I'll have a look, cheers
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Re: extremely cr#p brakes

Postby max midas » Sun Aug 07, 2016 8:34 am

easy to test.
Disconnect the tube between the manifold and servo, see if the manifold is producing a vacuum with your thumb over the end.
Then test the servo by trying to suck the air out and listen for the diaphram clunking up and down.

My Midas' brakes are great at speed, but could do with being a bit sharper when coming to a stop, I suppose there's lots of vacuum at high revs with the throttle closed, much less at tick over. I did wonder about creating a consistent vacuum throughout the rev range by replacing the manifold vacuum with an electric vacuum pump as in electric vehicles. The price put me off!
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Re: extremely cr#p brakes

Postby max midas » Sun Aug 07, 2016 9:04 am

It's just occurred to me that if you have anything other than a standard cam, you'll have less vacuum at low revs.
Perhaps the answer would be something like this
http://www.ebay.com/itm/Brake-Vacuum-Ca ... 2068146991
You should get a good vacuum even at low revs.
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Re: extremely cr#p brakes

Postby benofbrum » Tue Aug 09, 2016 5:20 pm

could the hose be collapsing under vacuum?
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Re: extremely cr#p brakes

Postby ACourtney » Tue Aug 09, 2016 8:01 pm

Tony Moss had a similar problem around a year after fitting a new servo and new master cylinder. His local garage diagnosed that the servo wasn't working properly, so he got hold of a replacement and came to my workshop so that I could help him fit it. When we lifted the master cylinder off the servo unit, the reason that the servo wasn't working effectively became apparent - it was full of brake fluid :o

We came to the conclusion that the "new" master cylinder was new old stock that had been sat on a shelf for at least twelve years and possibly longer. The seals had probably become brittle during that long period of storage. My preference now is to fit new seals into master cylinders rather than replace the whole cylinder, as that way I get to see the condition of the seals. I now have a set of honing brushes that will clean up the inside of master cylinders to remove any rust, scores etc.

We took Tony's servo apart, which I must admit I hadn't done before and cleaned it all out. We lubricated the seals on the valve with red rubber grease and talced the diaphragm (stops it scuffing on the housing) then put it all back together again and I believe that it has been working fine ever since. I then learned that the service parts for the servo are the same as used on the later Minis, post 89 - http://www.minispares.com/product/Class ... 0to%20shop.

ImageMini servo kit by Alistair Courtney, on Flickr

The only difference being that the Mini kit comes with all the valve seals fitted to a new actuator rod, which has a screw thread, but the Metro actuator rod is shorter with a fixed clevis fork. With a bit of care and some red rubber grease the seals can all be transferred to the Metro actuator rod. The servo repair kit and master cylinder seals are now made by Caparo, who bought up AP a few years back. They list a Rover Metro/100 servo repair kit in their catalogue, but it is not a stock item and I haven't been able to get hold of it anywhere.
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Re: extremely cr#p brakes

Postby streetsy » Tue Aug 09, 2016 9:41 pm

Just a little thing to try, wedge the brake pedal down, perhaps from the steering wheel with a bit of wood, whilst the car is parked up. The constant pressure on the fluid helps any tiny air bubbles join together which are easier to bleed out and also tend to head for the highest point in the system which is usually the reservoir. It certainly works on my motorbikes and I always do it on cars for a few days or more if possible before I bleed the brakes.
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Re: extremely cr#p brakes

Postby Jin » Fri Aug 12, 2016 10:43 am

Thanks for the replies folks.
I've changed the servo hose to no avail.
When I pulled the hose out the servo there was a massive hiss of air indicating there were no leaks between servo diaphragm and one way valve mid way down the hose.
I did bleed some new fluid through a few weeks back but prior to this there has been no fluid loss in the years I've owned the car and the brakes have been poor from day 1, I've just took my time getting round to addressing the issue.

I don't suspect air in the system since I've bled it recently with no inprovement, the pedal is firm and even if there was air in the system the motion of the pedal may increase due to compressing the air but the force would still be the same pedal to pad.
I've a feeling the rears are poor so will be looking to rebuild my rear subframe sooner, but I don't feel this is all that is the issue.
Could it be because the pedals have been cut and shut?

Just an idea.

Thanks
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Re: extremely cr#p brakes

Postby ACourtney » Fri Aug 12, 2016 3:05 pm

It sounds like the induction side is drawing vacuum, but are the servo diaphragm and seals really holding it?
You need to put a vacuum gauge on it to see if you what happens when you apply the brakes. A vacuum tester/bleeder kit can be used for this. If the valve seal is leaking then the assistance will reduce as you apply the pedal.

If the servo is working okay then it may be a question of how well the disks and pads work together. As most of the weight (probabaly 60 to 70%) of a Midas is at the front then it is at the front where most of the braking needs to be done.
The problem with Metro (and Mini) brakes is that they are relatively small diameter so the pads only have a small leverage on the disks. Therefore the friction coefficient between the pads and the disks becomes more important. I once had a problem with getting EBC greenstuff pads to work on another car and was advised that my problem was that the pads were not well matched to the disks (the disks being those already on the car). A change to EBC's own disks improved the brakes no end. The brake manufacturers will select the pad material and brake disk hardness to provide the optimum coefficient of friction. If they are not well matched then the pads won't grip the disks and will probably glaze over.

The other thing is to bed the pads in properly. Some manufacturers suggest using the brakes lightly for the first few hundred miles and then gradually increasing the pressure. Other manufacturers suggest the more ferocious heat cycle method of half a dozen hard applications slowing the car from 60 to 10mph, but not actually stopping, then cruising for 10 minutes to let the brakes cool down. I have used the latter approach in the past and afterwards you can see that the disks have gone matt grey from picking up pad material.
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