Puzzled by Sinking Brake Pedal

ByronJ

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Hi guys. I would appreciate your thoughts on the behavior of the brakes on my 80. This is the 24v 1995 without ABS. While I was travelling in Greece Zelda reported that Zorro's (thats the 80 with the issue) brakes were 'a little soft'. I got back to Cape Town a few days ago and the brakes were indeed dodgy. When pressing the pedal the brakes were applied but the pedal slowly continued moving to the floor!

There is no sign of brake fluid leaking so I pulled the master cylinder. On inspection the inside of the cylinder looked mirror smooth so I ordered new seals for the cylinder (3 working days from Johannesburg). These I installed today. With the engine off (no servo) the brake pedal is rock hard (an improvement) but once the servo is working the pedal will slowly sink to the floor under hard pressure.

With the brake pedal fully depressed and the transfer box in high ratio I cannot move the car but in low ratio the car can be moved with a fair bit of creaking and groaning. I am certain the brakes are properly bled (I fit a long clear tube on each bleed nipple and raise its end to the car roof and all air eventually finds its way out).

My only thought is that the master cylinder itself is worn (despite its mirror looking insides) and thus oil is bypassing the new seals. I would appreciate any insights you guys can offer before forking out a small fortune on a new cylinder.
 

t1pper

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Aug 7, 2014
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Had this on a Mitsubishi L200 a few years ago, turned out to be a seized caliper not moving properly making the pedal soft.
You probably are not feeling the problem without the servo assistance.
I blanked off each output from the master cylinder in turn until I eliminated the fault.
If all outputs are blanked and the pedal is soft time for a new cylinder
 
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moggy1968

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Im with your theory, can be leaky seals in the master or slave allowing fluid to push past
 

Richard Turner

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Check behind the rubber boot of the load sensing valve for leaks, also, it has a bleed nipple on it.
 
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MarkW

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You sure you’ve not been driving down mountains too fast again Byron?

If it was me I’d start with Re bleeding everything again. I had a nightmare with the track Car last year where it just wouldn’t bleed properly until about the 4th try.
 

Higgy

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You sure you’ve not been driving down mountains too fast again Byron?

If it was me I’d start with Re bleeding everything again. I had a nightmare with the track Car last year where it just wouldn’t bleed properly until about the 4th try.
I had the same trouble on my old 95.. I changed the master cylinder, but it was just the same. it wasn't until id bled it up half a dozen times that it finally came good. Like Mark Ws post. They can be a bugger sometimes
 

ByronJ

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Thank you all for your input, it is much appreciated. I had another bleed all round last night and the brakes improved, the car stops fine but the pedal just does not feel right to me :grimacing:.

Zelda went out on a test drive and felt the brakes are better than they have ever been! Perhaps I am just paying much more attention (over sensitive) having pulled the system apart?

Anyway we are off on a week trip up north this morning so fingers crossed. All tools fluid etc. packed just in case...

I will have another look at the system when we get back. Thanks again for your thoughts.

Byron
 

ByronJ

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You sure you’ve not been driving down mountains too fast again Byron?

If it was me I’d start with Re bleeding everything again. I had a nightmare with the track Car last year where it just wouldn’t bleed properly until about the 4th try.
Last night I reversed into the garage (knowing the brakes were poor) and couldn't stop. Managed to brake Zelda's huge dance mirror :fearscream:. Going to have to replace that pronto.

Mountain test coming later today...
 
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Jake the Peg

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You could try clamping the flexi pipe to each caliper in turn and see if you get an improvement, then you can trace it to a single point, if all 4 are clamped and you still have the the same feeling I’d look at the master cylinder..
I’ve just done a repair on an L200 with a similar fault, seems previous owner fitted new front pads, but one of the shims on the rear of the pad was displaced and was pushing the piston back all the time,

I sometimes wonder about people doing maintenance on safety systems without the correct knowledge and competence!!
Cheers, hope this is of some help
 

Jake the Peg

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By the way I saw a post on a motorcycle forum I’m on, where the guy had taken his rear wheel off for a tyre change, rebuilt it with the pads together and the disc between the rear of the pad and the caliper, he wondered why the wheel was so hard to get lined up and why the brakes were noisy, this was on a 12k bike just a few months old!
 

IRLGW

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Sounds like a weeping seal on one of the pistons. If you have the know-how, make yourself a brake pressure tester by connecting a hydraulic guage to your bleed nipples and see what pressure you are getting (Specs in FSM) you could also just pull each caliper one by one and lock it with a piece of wood while pumping the pedal and do a visual. As you have changed the MC I would pull a half-litre of new fluid through each corner to make sure you have removed the air.
 

frank rabbets

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Byron I take it yours is a Diesel engine? I have always had slowly sinking pedal with engine running. I had the car when 3 years old and noticed this. I changed seals recently for genuine Toyo ones but it still does this. However the pedal does not go to the floor but stops about one inch off. Pedal is solid with engine off. Air will not give a solid/sinking pedal but a spongy one. If your pedal is high/solid with engine off you don't have air in the system. Are you sure your pedal eventually goes ALL THE WAY TO THE FLOOR?

My "problem" has never bothered me. There is never a situation on the road where I would have to force the pedal for such a long time, as the car would be at rest. The diesel engines have an air pump as there is no inlet manifold vacuum that petrol engines can rely on. Maybe there is some connection.

If you have 2 outlet pipes ( front and rear) on your cylinder the problem must be in your m/c as there is no way fluid could get back to the reservoir with your foot on the brake pedal. I assume you have no leaks.

P.S. I 've had brake fade in France when coming down a mountain in France in 5th gear. That is nothing to do with faulty master cylinder or brakes, it proves they are working. I've had brake fade on every car I've owned except my Utima. Must be my driving.
 
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clivehorridge

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My money's on the master.

Once at max pressure (emergency stop type pressure) the pedal should not move any further, assuming no external leaks.

Bleed, bleed and bleed again....

Then buy a new master cylinder...
 
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Higgy

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My money's on the master.

Once at max pressure (emergency stop type pressure) the pedal should not move any further, assuming no external leaks.

Bleed, bleed and bleed again....

Then buy a new master cylinder...
I like that Clive... I bought a new master cylinder and still had to bleed bleed and bleed again... Bleeding thing!
 
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Jake the Peg

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If you do replace the M/C again I’d suggest bleeding just one corner at a time, that way you know when each caliper is good and you aren’t guessing where any issues are. Start at the furthest away from the M/C and use short pumps of the pedal until you see good fluid movement at the bleed nipple. Mak sure the reservoir is spotless clean before you add any new fluid, and use new fluid from a sealed container. Brake fluid absorbs moisture so keep lids on unless you are filling up

Cheers
 

Towpack

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Forget the pedal pumping method. It’s hard work and problematic. Get a brake bleeding device of some description. I’ve used the Gunsons version, which pressurises the MC and it makes bleeding a quick and easy one man operation. There are also some vacuum bleeders which such the fluid through from the nipples which do an equally good job. I inadvertently ran my MC dry when bleeding mine but I just refilled it and went round all the nipples again in just a few minutes.
 

ByronJ

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Jul 7, 2012
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Please accept my apologies for the delay in returning to this forum but our NS knuckle steering arm parted company with the steering knuckle in a remote area to the east of the Cederberg which involved a bit of an adventure (including hiring another 4x4) to sort out. I will post separately about that.

Thank you all for your input, I really appreciate your thoughts. After more investigation I believe the current situation is thus:

- there are no leaks of fluid from the brake system
- all slave brake cylinders are operating normally
- I am almost certain that there is no air in the system
- The car stops well and I can lock all 4 wheels (there is no ABS fitted).

The pedal still slowly sinks under pressure when the engine is running. However I think @frank rabbets has described my situation to a T.

"Byron I take it yours is a Diesel engine? I have always had slowly sinking pedal with engine running. I had the car when 3 years old and noticed this. I changed seals recently for genuine Toyo ones but it still does this. However the pedal does not go to the floor but stops about one inch off. Pedal is solid with engine off. Air will not give a solid/sinking pedal but a spongy one. If your pedal is high/solid with engine off you don't have air in the system. Are you sure your pedal eventually goes ALL THE WAY TO THE FLOOR?"

My engine is diesel and having checked closely the pedal does not sink ALL the way to the floor, the pedal is rock solid with the engine off. With the engine on the pedal does sink under pressure but once the pedal has stopped sinking there is still a great deal of force being applied by the brakes. I am now of the opinion that my brakes are fine. In fact once we fixed our car Zelda refused point blank to drive our hired 4x4 back out of the desert (plenty of steep rough tracks) as she said she felt much safer in the 80 :grinning:.
 
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frank rabbets

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My brake pedal stops about 1 inch off the floor in neutral with engine running. A lot of diesel cars have this case but I have not sorted out the cause in my head yet. Glad your mind is at rest.
 
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ByronJ

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I am in wales
Jul 7, 2012
349
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Forget the pedal pumping method. It’s hard work and problematic. Get a brake bleeding device of some description. I’ve used the Gunsons version, which pressurises the MC and it makes bleeding a quick and easy one man operation. There are also some vacuum bleeders which such the fluid through from the nipples which do an equally good job. I inadvertently ran my MC dry when bleeding mine but I just refilled it and went round all the nipples again in just a few minutes.
Hi Towpack. I am in agreement with you in using a simple bleeding system. The method I use is very simply and cheap.

I attach a metre or so of clear plastic tube (ID 6mm) to each bleed nipple in turn and tape the top of it to the side of the car so it is well above the height of the brake master cyclinder.

Then I fill the cylinder with brake fluid and allow gravity to do its work.

Bleeding brakes IMG_5869.jpg


You can see bubbles making their way out in the above image. Eventually all air is removed and I lock off the nipple and move on to the next one. There is usually a small amount of fluid that leaks from the nipple threads but as there is always positive pressure being applied by the master cylinder reservoir no air is introduced.
 
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