OB,

My logical head says that if your pedal goes to the floor, it must be caused by either:

- failing m/c seals
- a leak of fluid on each press
- air SOMEWHERE in the system

And that's the order you have to investigate them to sort this problem. Don't guess at where the problem might lie. There's a logical order to nailing this problem.

So, starting with the first one, if I read correctly, you said that you plugged both ports on the m/c and then you were able to achieve a rock-hard pedal when the engine was running.

A couple of important questions:

1. did you press the pedal to test this AFTER the engine was running? You musn't push your foot, or have it resting, on the pedal until after the servo vacuum has built up. This is because it's just possible to get a lock of the seals under non-assisted pedal pressure which can then continue to hold as servo pressure is applied. Whereas if you start with your foot off until the engine is running , all of the m/c piston movement will then happen under servo pressure and really test the m/c seals.

2. was there almost no movement whatsoever before the pedal was hard? With the ports blocked, there will be an almost imperceptible movement of fluid, so there should be almost zero pedal movement.

If you can't confirm or remember the results (and you're not going to like this!) you have to go back and do this test again. Until you can confirm that the m/c works properly with blocked ports EXACTLY as I've described above, you're wasting your time doing anything on bleeding the wheel circuits.

I'll take the needle off the stuck record now!

Cheers

Paul

 

Hi Paul

I will go back and do the Test again, although I'm also thinking that it could be the Seals on the Master Cylinder like you say

Unless I'm missing something, it doesn't look that difficult to take off (just a couple of nuts) so wondering if it might be a good idea to take it off and see if it is leaking and even put new Seals in before I put it back

Or Plan 'B' that would be my preferred option (since there are only Two things left that it could be!) (1) Master Cylinder (2) Brake Servo

That Master Cylinder looks a dead ringer for a Land Rover one and even looks the same Casting, so if I take the Master Cylinder off, I will be able to do a better comparison and then if its the same then I can buy a new one

Same with the Brake Servo a 1980 XK8 one looks the same and while I wasn't planning to spend that sort of money, sometimes that turns out the better option, as otherwise it could take me for ever to fix it and would still be cheaper that taking it to a Garage not that I could do that anyway

At least with New Components, it won't be 'hit and miss' so seeing as I am selling this Car, I'm all for spending whatever it takes to Fix it

As I'd also like the New Owner, who may even end up on this Forum, to know that I went the extra mile and spared no expense when it came to making sure that he would be buying a really nice car without any unknown problems or anything covered up

Where is that emoji with the 'Halo!'

 

Even if a master cylinder looks the same, you need to verify that the bore and stroke are the same. Any difference will affect braking. The same goes for the servo.

You basically have a standard pre-ABS brake system now since you removed the ABS system. Since that had zero effect on your issue, plan to reinstall it before selling. The car will be worth a lot more with it than without.

Please give a detailed explanation of exactly how you are bleeding the brakes. Frankly, I'd suggest getting a power bleeder. They are pretty easy to make, plenty of DIYs on YouTube. I made my own out of a plastic handpumped sprayer, and it is by far the best way to get air out of the system. It also refills the master reservoir as it bleeds, so you can't get air in the system.

Jon

 

Hi Jon

I'm running a long continuous tube from each of the Calipers in turn and then using the Brake Pedal to Pump Brake Fluid back into the Reservoir

This has been working amazingly well and has eliminated all the Bubbles not only in the Calipers but also in the Brake Lines

So this also refills the Reservoir as it Bleeds the Brakes, where being a Closed Circuit, there is no way that any Air can get in

And so it looks like the Master Cylinder could well be the Problem and if not that the Brake Booster

I'm not being lazy and taking the easy way out, as I have already spent 3 months trying to fix it and trying to eliminate any problems I have come across, so while I am quite willing to try and rebuild the Master Cylinder

It may be more cost effective in both time and money to replace the master cylinder with a new one

 

Alex
A couple of points: a 1980 XK8 does not exist!

Seriously though, how are you preventing the air bubbles in your circuit from returning to the caliper? Are you using a helper or a non-return valve?

 

Hi Greg

The Air cannot return to the Caliper as the Tube that goes into the Reservoir is always Submerged in Brake Fluid and the Reservoir was always kept topped up and now the whole system including the Pipes are 100% Bubble Free

Everything was working ok before I replaced the Front Calipers, so before I get a New Master Cylinder there are a Couple of things that might be worth Checking

(1) I double checked to make sure that I hadn't put the Front Calipers on upside down

(2) When I was Bleeding the Rear Brakes, I noticed that the Rear Bleed Screws, looked a little bit rusty so while they were keeping Brake Fluid in, they may have been letting Air back in as well Do you know what size the Rear Bleed Screws are so I can get some new ones as it could be that

(3) Maybe it could be the Rear Proportioning Valve? I heard that you can get a Tool to temporarily re-set them while you Bleed the Brakes

(4) Maybe one or both of the Rear Wheel Cylinders have got stuck and is letting Air in and so may need replacing

 

OB,

If you bleed brakes on your own, even with good intentions, you can not see what's coming out of the calipers. You really need 2 people. There's no easy way around this. Otherwise you are literally guessing on what's actually happening at the caliper outlet.

Seriously, don't even think about removing the m/c, changing it, or doing any further wheel circuit bleeding. Just plug the m/c ports, do the test properly, and then you'll know where the problem lies. Otherwise you are constantly throwing paint at the wall and hoping some of it will stick.

I also agree with Jon that you stick the abs before sale. I'd be nervous about buying an XJS with the ABS disconnected and removed. I hope you left all the fluid in it and plugged all the ports when you removed it!

Cheers

Paul

 

Alex
Further to Paul's reply about brake bleeding:

1. When you depress the pedal, air and fluid, rather like a passenger train with a few freight cars (air) in between the coaches (fluid) gets shunted down the pipe.
2. If you then lift the pedal WITHOUT shutting the bleed nipple first, two things happen
3. one, the "train" reverses direction passenger cars and freight cars included, and
4. two, extra air also gets sucked back into the caliper down the still-undone bleed nipple threads.
5. points 3 and 4 above are NOT changed even if the end of the bleed tube is in fluid, as the air "train" is just getting shunted up and down the "line" albeit on a circular track!

To bleed the brakes properly this is the procedure:

• One person on the brake pedal
• One on the bleed nipple.
• flexible clear polythene line on the bleed nipple going up in an arc and down, so you can see the bubbles at the top of the arc.
• Shout out to pedal person "PRESS and HOLD" and loosen bleed nipple at the same moment the pedal is pressed
• Pedal is held down
• Bleed nipple is tightened
• Shout out PEDAL UP helper releases pedal, thus introducing fluid into the M/C from the reservoir rather than sucking it back up the line
• Repeat steps until no more bubbles in the arc of poly tube. It does not matter where the expelled fluid goes, as long as the nipple is closed before the pedal is released
• After 4 pedal strokes, having tightened nipple, check the reservoir level.

 

Hi Greg

I'll give it a try and see if it makes any difference but its usually dark when the GF gets home, so maybe this weekend

 

Hi Guys: Paul (ptjs) Jon@Greg

Apologies all round!

No excuses, You were right and I was wrong, its just as Simple as that and so you can Congratulate yourselves on Fixing Stupid!

Although Greg's explanation was the one that finally Sunk in, as I read that over and over again until I suddenly got it!

And I've got a Pressure Bleeder, that I hope will help to do the Job and so as soon as it stops raining, I will try again and hope it works!

Alex

 

Hi Guys

I've finally managed to Fix the Brakes!

The Brakes are now Perfect Engine Running or Not, Rock Solid with no Sponginess whatsoever and also nothing to do with the Master Cylinder or in the way I was Bleeding it

My MOT Tester who has seen it all done it all read the Book and got Multiple Tee Shirts gave me this 5 Minute Fix on the Phone and it Worked, although to be quite honest, I am not sure how or even why

And he's also got the gear to Fix the ABS which he said he would do for £60 if I drive it over, as soon as I've got some New Tyres and have got her Insured

All I know is that it Solved the Problem that loads of People on YouTube and that includes myself, up until a few minutes ago have been unable to Fix

 

OB,

Excellent! Great news!

I'm sitting here with bated breath! Do tell us what was the answer?!

Paul

 

Hi Paul

Ok and here's what happened!

Many many Sleepless Nights trying to solve the problem with the Sinking Brake Pedal, that as you know worked perfectly fine until I Started the Engine, at which point the Pedal would go to the Floor with only the lightest touch

What made things worse if they could get worse, was that loads of people on YouTube had exactly the same problem and they couldn't fix it either!

So I'd more or less resigned myself to buying a New Master Cylinder about £300!

But before I threw any paint at the wall (Lol) I thought that I would have a word with my MOT Tester, who has the best equipped Workshop that I have ever seen and 30 Grands worth of diagnostic equipment!

When as soon as I told him the problem, he snapped right back with the Answer almost before I even had a chance to say Thank You and Goodbye!

Woah, Woah, Woah, hang on a Mo, are you sure that's it and what if its not?

Do what I've just told you and then drive off into the Sunset!

It maybe just a Hobby to you but I do this for a living and I come across stuff like this all the time, so its not a new one on me and also from what you've told me, there is nothing wrong with your Master Cylinder

And as far as Bleeding it, you've done exactly what I used to do and Bleed it on a Closed Circuit back into the Master Cylinder and your idea with the long Tubes, was if anything even better, as if you can't see any Bubbles, then there isn't any Bubbles, Engine Running or Not

So do what I've told you then call me back and tell me that you've fixed it

So getting back to the Problem together with the Fix one of the Slider Bolts in the Rear Calipers was Seized and I didn't even know what a Slider Bolt was!

What was apparently happening was without the Engine Running, the Pedal felt Solid but as soon as the Engine was running, the Brake Booster pushed hard enough for the Pedal to go down and while I can't pretend to know exactly why that was (although you might!)

All the Slider Bolts needed was a bit of Wire Brushing and a bit of Silicone Grease and then once having done so I Started the Engine and then when I touched the Brake Pedal, Engine Off or Engine On, this time it felt Solid as a Rock!

As the light was fading I'd only done one side, which as it turned out was fortunately the side that had the problem, although I'll do this other side Tomorrow

But as for me I'm still in Shock that I've managed to fix it, which only goes to prove 'You can Fix Stupid' being me!

So I'm seeing a Bottle of Single Malt, in my MOT Testers Future!

 

I'm not going to begin understanding how a stuck slider bolt caused your problem. But I will give you this bit of advice. Don't look a gift horse in the mouth!
Glad after all this it appears to be well on it's way to being fixed 100% Good Job. You never loose till you quit trying.

Jack

 

Hi Jack

I don't really understand it either, maybe it was jamming the Caliper Piston so that it wouldn't slide back and was maybe letting Air in and I don't ever remember this problem ever appearing on the Forum, so maybe that was why a fix was so hard to find

If I'd replaced the rear Calipers then it probably wouldn't have happened but they are Crazy Expensive at £126 each and everything was working ok until I changed the Front Calipers, so that was why it never occurred to me that it could have affected the rear ones

But if it ever goes wrong again, then that will be the very first place I will look and I would also change the rear Calipers if I had to but as for now I'm still in shock that I managed to fix it, just plain Lucky I guess

As although it was a Bank Holiday, I've got my MOT guy on my speed dial, just in case I ever need to be rescued, as in an XJS you never know!

 

Interesting. The fluid must be going somewhere and it is not leaking out, so I wonder what the exact mechanism is. It is always possible that a flexible in the rear axle flexibles set is expanding under pressure and can only do that under servo effort. If the caliper is stuck, see next para, this cannot be discounted.

As far as I understand it (and I have these on my Ford Mondeo) calipers with slide pins have only one piston, and when it is presurised the piston's action acts two ways, in effect as a double ended piston: 1: it pulls the pad/caliper in towards the disc on the non-piston side and also, 2: pushes the pad the piston acts on directly into the disc on its own side.

If the pins the arrangement slides on become stuck so it cannot slide, maybe, MAYBE, the piston can move but runs out of travel as the master cylinder limit is reached, or maybe it replicates pad "knock off" whereby if the axial play is greater than the piston maximum travel, the pedal goes to the floor as the pads are still too far from the disc to make contact? And MAYBE you need servo effort to make this occur; i.e. a on an unservo'd master cylinder the pedal feels hard because the piston/slide arrangement needs servo for this to happen.

When disc brakes were first used in cars they were used exclusively in racing, where the brakes are used every corner (ie every few seconds). When they put them in road cars, the testers found that they often had to pump the brakes to get them to work. This was because rear axle in/out lateral tolerances allowed the pistons/pads to be progressively knocked back into the caliper as un-braked curves and corners were the road norm. Such that eventually the gap the pads had to make up was greater than the available pad travel from the master cylinder could move the pads. Axle tolerances were reduced to 1.5 thou to cure the problem, which of course, did not affect drum brakes.

 

Hi Greg

Your analysis sounds 'Spot on' as they are only Single Piston Calipers on the Rears but Double the price of the ones that I put on the Front, so definitely sounds like a Piston replacement could be on the Cards for these, rather than New ones like I put on the Front

These are also hidden behind the Brake Discs and seem like a bit of a Chinese Jigsaw Puzzle, though not as complicated as they look and no signs whatsoever of any Fluid leak but after what you've been saying, replacing the Disc Brake Pads, sounds a good idea!

So my Plan is multiple Test Drives round 'Orange Blossom Towers' and then I can fit some New Brake Pads when I take the Wheels off to put on new tyres

 

Hi OB,

Just wondering (after reading Gregs post detailing the procedure for bleeding the brakes) whether your problem was simply due to ineffective bleeding, resulting in air constantly being in the system. If you didn't tighten off the bleed nipple after depressing the pedal each time, I think it is likely that air will have been drawn back into the system.
Just as a matter of interest, did you bleed according to Gregs bleeding procedure before the problem went away?
Great that it's sorted whatever the reason and, as they say here in Ireland 'Fair Play' for your persistence... many people would've given up a long time ago!!

 

Hi Paddy

No I didn't do that, instead I Bled the Brakes with my own Closed Circuit System, that produced a Rock Hard Pedal every time, where the Brake Pedal only collapsed when the Engine was running

So while I got all the Air out,it was something else that was causing the problem, which in this case was stuck slider pins, that would never have occurred to me as being the cause of the problem, where as soon as I'd freed those off, everything worked like a Charm

Its only taken me 3 months to find the cause of the problem but just a few minutes to fix it!

Except you can't fix anything, when you don't even know what you are supposed to be trying to fix and if that doesn't drive you Crazy than I don't know what will, though thankfully my MOT guy was all over this and knew what was wrong

 

OB,

I'm still very confused by this. Sticking slide pins on a caliper nearly always result in a dragging brake, to the point of seizure. I've had this twice on my current Range Rover over the years, which is nearly always caused by lack of mileage. I also had it on one rear caliper on my 4.0 XJS a few years ago. The caliper tends to slide in, but can't then slide back out because of corrosion on the pins, and the pads eventually get locked in tight to the disc, causing squealing, brake dragging and heat buildup. Being realistic, it's more likely to be the cause of a sticking caliper piston, but the same principle applies that it could be the sliding pins.

However, it seems that the only way you could have a sinking pedal under only servo-assisted pressure, that is caused by stuck sliding pins, would be as follows:

- The caliper pins were stuck with the outside part (non-piston side) of the caliper pushed away from the disc. Therefore the pad was not touching the disc on that side.
- Under normal pedal pressure, the piston is pushed out to make pad contact with the disc (on the piston side only).
- The pedal is then hard
- With the boost pressure applied from the servo when the engine is running, there is then sufficient force applied down to the piston and pad to deflect the disc to take up the free space to the other pad on the non-piston side.

In my head, that's the only way the pedal could sink under servo pressure but not normal pedal pressure. The servo pressure is causing the disc to bend slightly which normal pedal pressure can't achieve, and there's a small gap on the non-piston side where the caliper and pad is stuck retracted back from the disc.

But my confusion is:

How would the caliper and the pad, on the non-piston side, have got pushed back from the disc before the pins then got stuck? It couldn't have been when the car was last driven as the pedal would have been sinking to the floor then.

So, maybe the disc is slightly distorted and the runout has pushed back the caliper pad, maybe when the car was rolled forward many years ago and the brake pedal then not subsequently pressed. Then the pins seized whilst the car was immobile for many years? Is that possible? And therefore maybe the reason why we've never heard of it before, as sliding pins don't tend to get stuck in the "outward / open" position. It would take a strange set of combined circumstances, as suggested, for that to happen.

So, I would definitely now jack the whole car up (LSD remember!) and turn that back wheel and carefully check if there's any tiny disc distortion. If so, I'd change the disc (and pads).

As regards how you could have diagnosed this earlier; I hate to say it but if you'd followed the routine of blanking out each m/c port separately, you'd have eliminated the m/c as the cause and pinpointed that the fault lay somewhere in the rear circuits.

BUT, the great news is that it's fixed! And we've all leant something! (Even if we're still slightly confused as to how it happened!)

You must genuinely be so relieved! I'm sure the single malt is still flowing! (Then go and check for any disc distortion)

Cheers

Paul