Sinking Defender break pedal... No fluid loss!?!

Jon uttered summat worrerz funny about:

Check the servo hoses. I had a pipe chaff through on my last RRC which displayed similar symptoms. Worse when the pipe was warm as it leaked more... it only got warm on a motorway run which provided some interesting entertaiment and brown pant moments when coming off at a junction.

Lee

Or master cylinder seals, possibly?

David

Master cylinder seals, possibly slave cylinder sucking air in - vacuum loss usually makes the pedal go rock hard.

Richard

Thanks for the reply's all. Looks like I'll check the hoses and then get to grips with the master cylinder...

Looks like another joyous weekend! Still, it keeps me out of the garden!

Jon

My money is on either the master or slave. Fluid is bypassing the seals on one or the other - hence no loss. Classic symptoms.

master cylinder seal leak sounds about right to me if its the slave it would be dripping past the pistons.One other alternative to consider if you have had a VERY enthusiastic day hot brake fluid can leave air in the system resulting in a spongy pedal which compresses depends how old the fluid in the system is and whether is has been bled properly at service. Derek

Hi Jon I agree with Richard, every servo leek I have come across makes the pedal go hard. Try clamping the flexi hoses with vice grips ( or flexi clamps if your posh ) if the pedal still drops suspect the master cylinder, if it is firm release one flexi at a time and try the pedal to indicate where the problem may be.

Hope this helps

Icky

Brake fluid is newish - was replaced about 3000 miles ago.

Brakes were fine so it only just started.

Day wasnt that enthusiastic and the fluid certainly wont have boiled to induce air.

Will check the master cylinder tomorrow...

Finally got around to looking at the brakes in a bit more detail last night, but only for a few minutes.

There is still not loss of fluid in the resevoir or signs of leakage around any caliper or flexible pipe.

BUT... I did note that the fluid in the two halves of the resevoir are clearly different colours!

The compartment furthest away from the servo looks like brand new DOT4 and is a vegitable oil / yellow colour. The resevour compartment nearest the servo however is a much darker colour. Could this be stained with rubber if some internal seals have given way??

The brakes have not deteriorated any more but I have found out that I need a runway to stop from 70 - 80 mph! The feeling is just like when fluid boils...the more you press, the more aware you are nothing is happening!!! I can however say that the fluid has not boiled.

Jon

More likely that the system contains two different oils, it is possible that the oil could have carbonised. I have had this happen on a tractor with internal wet brakes but I didn't think dry brakes could transfer enough temperature through a caliper.

The symptoms are typical mastewr cyclinder seal leak, as has been pointed out in earlier responses, what happens is the seal leaks a bit around the side under mild pressure but when you stamp on it the extra pressure "balloons" the seal against the wall to reseal it.

AJH

On or around Tue, 23 May 2006 08:13:51 +0100, Jon enlightened us thusly:

you've got dual circuit brakes, sounds like one circuit malfunctioning.

and BTW, what you describe is brake fade... fluid boiling results in the pedal going to the floor and doing sod-all, due to vapour not being compressible in the same way as fluid.

Well I managed to get a new Lucas master cylinder... Low and behold the local LR garage was the same price as my only other quote (sorry Richard) and most convenient.

Problem is that now Ive got the new master cylinder on and brakes thoroughly bled with new DOT4 I STILL have the same problem.

Flexible hoses wouldnt baloon enough under pressure to induce a sinking pedel would they?

Need to do a bit more driving to test out but it really has me baffled now!

Are the TD5's known for air locking when bleeding? I got quite a bit of air out and flushed a lot of fluid through after the last air at each wheel so Im sure under normal circumstances there shouldnt be any air in the system.

If the vacuum motor thing had packed up that wouldnt give me my symptoms would it?

Hopefully someone can shine a light on whats fast becoming a pain in the ar5e car!

Jon ...should have stuck with the TDi

|| Well I managed to get a new Lucas master cylinder... Low and behold || the local LR garage was the same price as my only other quote (sorry || Richard) and most convenient. || || Problem is that now Ive got the new master cylinder on and brakes || thoroughly bled with new DOT4 I STILL have the same problem. || || Flexible hoses wouldnt baloon enough under pressure to induce a || sinking pedel would they?

If the pedal is just spongey, it's air still in the system. If the pedal actually sinks to the floor, you've got a leak somewhere, with fluid bypassing the pistons. One good diagnostic is to get a hose clamp and clamp off the rear flexi on the back axle. If the problem is still there, you know the problem's at the front. Clamp off the front hoses in turn. If there's still a problem after that, then it must be the master cylinder. Not recommended if you've got braided hoses, though.

Thanks Rich, Time to get out some clamps!

Jon

I have only had a flexible balloon once ( On a Morris1000 ) that turned out to be fluid running between the layers once you are looking for it the problem is visable as the pipe flexes unlikely but go with Richards suggestion clamp the pipes off to eliminate where its happening. Derek right its not raining I have the ep90 its front diff time.................