Front brakes not releasing

[Forkie]

Taking it over this afternoon. Luckily i already had a set of new seals in storage so he is going to assume a strip down and have a look see. I thought maybe a rare occurence, as no one else seems to of experienced it, but that is how it appears, to me anyway, an imbalance between each half, on both calipers.
I did check through my bill pile and i did exchange them with CW, so at least i can assume they were built up correctly at the time. I will post up the findings. He is a keen chap with mysteries, likes solving them so no doubt he will be out on it this evening....

[Forkie]

More curiosity - i know some may be horrified at the thought of driving it in this way, but early afternoon was chosen as the quietist time, we are only 6 miles apart, and we agreed that i would pull over often to check for heat to the wheels, and coupled with bare minimum use of the footbrake, we figured that sounded a half decent plan, and assess things again if problems arose.
Well, no problems arose, and strangely enough barely any heat built up at all - and they felt no warmer than they ever did when i was feeling for hot wheels last year when first returned to the road. So i have a strange feeling that these brakes have been like this since the start, and hence keeping my top end capped. I stupidly took the train of thought the brakes are working fine, as the wheels are not cooking !!! Time will tell.

[Forkie]

Bit of an interim update. Bit of head scratching going on at first, was not just me!! He has removed OS caliper and stripped it - no corrosion at all, all clean - but what he did find behind each piston was quite a lump of the white fitting grease - assume that was used on the rebuild - too much it would seem !!! Rebuilt it, re - fitted it - kind of eased things a little. I then turned up at this stage, so putting both heads together the assumption was made that the system is holding pressure - not releasing itself, but will gradually after a period of time. Undoing the pipe to the Master Cylinder eases the issue. It appears the servo is fine so his theory is something is amiss within the M.C. - so thats the next strip. I re- sealed it just after coming off the road so either i did something wrong, seals gone hard, or he did suggest a weak spring - which i noticed that is another item CW does not list
So Monday - strip and clean re-seal NS caliper , strip and inspect MC. My only concern is the condition of MC , as they appear N/A , along with the spring. So hoping its just a seal issue....

[johnnydog]

If you do need a spring for the master cylinder Nige, I should be able to lay my hands on a good used one...

[Forkie]

Hi John
I most appreciate that - at this stage i am not sure whether to accept it, or not.....i do not wish to rob your stocks if i do not need to!!!! I will certainly bare it in mind....
Whilst taking some time out from the car - mow the lawn instead, it's far easier - obviously i have known for some time that the Mk2 MC is now N/A - what i do not understand is whilst CW lists N/A for the MC, the servo, any parts for the servo, and no listing at all for the spring, another Triumph dealer - name undisclosed - lists all of those parts - but again, no spring - but quality unknown.
In your opinion - if it ends up my MC is past it's best - what are my options???
Regarding said spring - is there a set length it should be, just so we can check mine? I am thinking if mine is within the set length, it should be sevvicable.

[Forkie]

Another small update - he thinks the issue might be the check valve at the front of the MC - unit itself looks fine, no scoring or corrosion, i had fitted the seals correctly many years ago....he is struggling to find a good diagram to show how the front end is supposed to go together, wants to make sure its all as it should be fitted. I am not at home at the minute to see if the big Workshop manual shows a better image, or can anyone explain how it is meant to fit together? I cannot picture the bits , was a long time ago i had it apart.

[Forkie]

Fixed by default!!! So stripped the MC as stated. Rebuilt it as he found it, as i would of done many years ago, and obviously to the diagram in the workshop manual. What the diagram or text does not make clear in our opinion , in the case of the washer behind the cup is slightly concave - in sheer desperation as he still found nothing wrong after all the stripdowns, he simply reveresed that washer, and it solved the problem !! Brakes are now backing off. A very very strange one indeed.

[johnnydog]

Well, after all these years and a good number of kits / seals fitted to brake master cylinders, I've never paid much attention to which way round that washer sits (or should sit!). Whether it's just been good fortune, but I've never had an issue with any I've reassembled - can't imagine that the washer will have been the same way round everytime, and I don't really see why or how it should make a difference, but at least it's a good result Nige!!!

[Clifford Pope]

Indeed, a good result, whatever the reason!

Just to summarise, the possible reasons for the pistons retracting are:

1) The angle of the seal recess in the slave cylinder distorts the seal, which on release of the pressure pulls the piston in a fraction
2) Normal very slight wheel bearing play causes the caliper to wobble, which pushes the pads and therefore pistons back
3) They don't retract - the friction of the discs in constant light contact with the pads does wear them constantly, but not very much
4) The master cylinder rod is pulled back by a spring, which reduces the pressure, so the hydraulic action is reversed
5) The rotating disc creates a boundary layer of air trapped between the pads and disc which pushes the pads and pistons in slightly
or
6) - a late contender - the washer in the master cylinder has to be the correct way round.

This last explanation is the secret of the whole process, but so vital to correct brake set up that none of the service manuals mention it, and it has only been discovered now, by accident, in 2024, 50 years after the car went out of production.
I don't think!

A contary problem reported by a Lotus Elan owner was of a mysterious over-retraction of one piston alone. The suggested explanation was that his seal was gripping the piston too tightly, so distorting more than it should in comparison with the others. It seemed that the degree of shininess of the pistons is critical.

This animation shows the distortion ofthe seal;
https://i.sstatic.net/epTU4.gif

However the slant of the groove in the animation is not in the place illustrated in the manuals, which is a puzzle.

[johnnydog]

So.......does the thin 'concave' washer fit 'concave' side to the 'open' end or towards the bottom of the bore then?
It's actually not 'concave' but 'curved', but 'concave' will suffice to describe its positioning!