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rear brake outside piston low pressure


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I have changed the rear calipers and master cylinder on my wifes discovery , but the outside pistons on both calipers are low on pressure the inside ones work fine but the out side ones barely move ?? but if you hold the inside ones in they work fine ? has anyone any idea how to solve this and also how do you refit the glass on the wing mirrors ive cracked two trying to get them on lol . .

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I have changed the rear calipers and master cylinder on my wifes discovery , but the outside pistons on both calipers are low on pressure the inside ones work fine but the out side ones barely move ?? but if you hold the inside ones in they work fine ? has anyone any idea how to solve this and also how do you refit the glass on the wing mirrors ive cracked two trying to get them on lol . .

Pistons that are not moving will be siezed and need freeing off

edit, didnt read it properly as usual, are they dual line calipers?

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they do not touch the discs , to the point the discs are rusty on the outside but not inside ?? they only move a few mill unless you hold the inside ones in then the go all the way to the disc ?

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due to the bias front to back, you'll find rear discs corrode quicker as unless you have a heavily loaded truck that you drive regularly, the discs don't get chance to get cleaned off. which reflects with the callipers, they just don't move as much as the front ones.

All LR callipers suffer, it doesn't take much resistance to stop a piston from moving freely.

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the pads are worn on the inside and untouched on the outside . i understand with the brake balance but its the fact that the outside pads do not touch the discs unless i hold the the inside pad in , which i cant understand surely that cant be right ?? give me a defender any day lol

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It sounds like insufficient flow going to the back. Are there any make up valves or anything in the line to the back?

If only the inner pistons move I don't think they are working at all, so you only have front brakes. They are only moving enough to clean the disk I suspect?

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like i said above the inner pads are worn ,the strange part is they are working ive tested them on a rolling road ? but only the inside . i dont be leave they are seized as when i hold the inner pistons in they move straight away when the pedal is pushed ?? as for the make they came from paddocks . could it have any thing to do with the master cylinder ?? a piston or seal losing or not producing enough pressure ??

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this is why im asking for help it doesnt make sense . the only thing i can come up with is not enough pressure to operate both pistons ? is this possible .. im going to change the master cylinder again to see if that is the cause .

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I have had my 1992 Disco 1 200TDi from new and do all my own maintenance. I greatly sympathise with St1g over this problem.

I have come to the conclusion that the rear brake balance valve is designed such that the rear brakes are rarely applied fully during normal road use and even when breaking is severe enough to ensure high pressure eventually passes to the rear callipers, the time that pressure is applied to the rear brakes is short and so the rear brake pads are in contact with the rear discs for only a short period. One piston in each calliper will always lead the other and my experience is that one side of each rear disc will be shiny and the other will corrode due to poor, or possibly no proper contact.

For many years I regularly towed a heavy caravan the length of the UK and during this time, with regular heavy braking, the rear pad wear was fairly uniform and both faces of each rear disc were clean and shiny. Nevertheless, one pad in each calliper was always significantly more worn than the other.

I didn't tow at all for the subsequent 7 years, mostly local use and short runs only, and I was soon back to the problem of irregular pad wear and unevenly corroded rear disc faces. I Have changed callipers once only and, in the heady days of available original parts, have renewed pistons and seals in the original callipers regularly, but the problem has always been there and I believe this is a Disco weakness that I have learnt to live with!

There is also a complimentary problem of fairly rapid external corrosion of the rear calliper pistons, compared with the front set, probably because they hardly move, which must contribute to stiffness and eventual seizure, which just exacerbates the problem of disc corrosion.

I have never changed the pressure control valve in the rear brake line, simply because the problem has always been there from new, and the symptoms have not changed over 250k miles. It may be that these valves can vary in operation, causing more of a problem in some vehicles than in others! We may just be unlucky!

I doubt your master cylinder is at fault. Take up caravanning? I'm towing mine again now and the rear discs have been clean and shiny for 30k miles on one set of pads. Albeit, one pad in each calliper is significantly more worn than the other. As usual!

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If there is a valve in there, I'm blaming the valve. The pressure is equal across a brake system unless it is interrupted by something.

My 10 pistons all get the same pressure because mine is an early RRC and not very technical. Same doohicky as later landy stuff in many ways, except they put more stuff in there so it did cleverer things.

Also, I know this sort of valve gear makes my 1991 transit van difficult to bleed the rear brakes. Maybe your rear end has air in it?

Dunno, but Marten's post is a good read :)

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been there have the t shirt same with our 300 disco

i came to the conclusion there is not enough volume of fluid moved by the master cylinder when some of the pads /discs are worn , new pads all round solved the issue for me

you say your inner pads are worn! try replacing them, which should decrease the distance the pistons on the inner have to move leaving more fluid for the outers

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I do believe that piston travel should not be an issue. The pistons follow the pads out as they wear and do not retract following breaking. The pads are in nominal contact with the discs all the time and there is very tittle "travel" when the brakes are applied. Just enough travel to "squeeze" against the disc and apply a braking force. New pads will not affect piston travel, just reduce the piston surface exposed to the elements. Level of brake fluid in the reservoir falls as the pads wear and will rise again only when the pads are renewed. Fluid volume displacement from the master cylinder when braking should be minimal and constant throughout the life of the pads.

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