rear brake imbalance - aaargh

Z

zebedee37

New Member
My 2000 TC has failed its MOT twice now!

The rear left caliper is hardly working.

Both calipers did work - but were binding badly - a month or so ago the seals in both calipers were replaced and the rear flexible pipes renewed. - On MOT day rear left hardly doing anything!.

Rear left stripped and the piston replaced. Re-test today and failed again!

Handbarke is balanced and works perfectly - so no issue withg the pads.
 
This will be a difficult one without seeing the car, but if I understand the sequence of events correctly all I can think of at the moment is the caliper has been reassembled without the washer under the plate. (The one held down with the allen bolt.)
I'm assuming both calipers are free on their pivots and are adjusted up correctly, and the piston was free in the bore and is not leaking. (And that it's a girling system)
 
Collapsed brake line? (The one connecting the two calipers)

Would restrict fluid going to the left side.

Dick West
 
Normally the collapsed hoses make the brakes less efficient, but also make them slow to release and hang on after you release the pedal.As the hoses have been replaced, I've assumed they're OK, but new parts can be faulty. Bleeding it through will tell you if they are.
 
bleeding is fine - ie loads of pressure!. The "plate" is in place.

Im totally flummoxed! and this is eating severly into my annual maintenance budget!
 
If it bleeds through OK it's not the hoses, and I take it the plate is fitted and retained with the allen bolt and the spacer washer between the plate and the caliper body? Looks like you'll be taking the caliper off again, so when you do, try and post some pictures of the internals BEFORE you start dismantling it. There is something here that's not right, and although I can keep making educated guesses I really need to see the caliper. If you've had the caliper off and stripped and rebuilt it already, post a picture of how it has been put back together internally. If you're paying a garage to do this, all the times this has had to come off after the first time should be FOC. Thinking about how someone could put it together wrong if they didn't know what they were doing is difficult because things can only go one way,but could the plate have been fitted upside down?
If you're a computer dunce like me and can't post pictures on the site, email them to me direct and I'll have a look.
 
Hi all,
Ive just had a brain rush!!!!!!!!! :p :p
Ive looked at a new spare rear caliper hydraulic piston I have, and a stripped caliper in the w/shop, and it seems to me that it could be possible that the piston was re-fitted upsidedown ??? so when you bleed its ok but the retaining washer is completely stopping the piston from moving up to press on the operating lever,as the rear calipers hydraulics operate on such small movements.
Also I once used the wrong type of brk fluid "citroen/rolls" and it caused the seals to swell up and sieze,fortunately it only got to the master cyl!! DOH!!!!!!! It took ages to work out what was wrong that time!!!!!!!!!!
I remember the 1st time I took one apart,it took me 6 cups of tea and the best part of a day to get my head round how it all worked ??? as it aint quite your normal caliper!! :p
 
I see where you're going with that, but surely you'd fit the seal to the piston the same way it came off, plus the h/b bullet sits with the point up so why would you fit the piston with the point down. Having said that it never fails to amaze me what some people do, I tend to give people the benefit of the doubt and assume just because I wouldn't do it neither would they, which is a bit unfair really. SHOW ME SOME PICTURES PLEASE!
If you fit the seal to piston the correct way, but fit the piston upside down, it will bleed through, but in use the seal will be useless. If you fit the seal to the piston the wrong way and put the piston in upside down it will maintain pressure but I cant see it working much if at all so maybe that's it.
Scariest thing with this is brakes are so safety critical that if you don't really know what you're doing, you shouldn't be doing them at all. (I include garages in that especially the fast-fits)
 
Its always easy when you know how,but when I first played with them I forgot how it all went back together,and had to study the manual pics & VERY hard to see diagrams. I forgot the retaining washer,but was extreemly lucky the plate held firm till it failed on the h/brake at mot time.
I went a bit wobbly when I found out how close I had been to loosing brakes ???
as you say,some good close up pics could help.
the only other thing I can think of is that there is a bit of sh#t in there causing an intermittent blockage to the piston.
 
I work on a lot of items which are one-of-a kind,antique,or just unfamiliar to me. I LOVE my digital camera for these items, and things like P6 rear brakes. I take plenty of pictures before and during the disassembly which makes up for my aging memory when it comes time to put them back together.

Dick West
 
Rear calipers were fitted with new seals and flexible pipes in the UK on one of my visits recently so I cant take back! Second Rebuild the other day was at a garage where the owner was pretty busy so I did some of the work whilst being "overseen" (well I took the caliper off!) Didnt take photos ! - But if I need to do it a third time will do so! ( We had an old caliper as a model whilst putting back together)

He's advised me to drive around a few days and keep pulling the handbrake to get adjustment better. I have jacked the back end iup today and put the car in gear - the rear left wheel turns under power whilst full braking! - Will put to retest again on Monday and visit church on sunday!
 
If the readings are fine on the handbrake and not on the footbrake I can't see how it can be adjustment. When they're not adjusted up properly the handbrake is the first thing to go, before the footbrake.I can only think something is restricting the range of movement of the hydraulic piston only, the handbrake tappet (bullet) is obviously working OK. I know I won't be popular, but I would say forget wasting your time driving it about, and get the caliper off and sort it before you run it back in for a re-test.You don't know what's wrong with it and it's only a single line system (I assume) and brake failures are a frightening experience, I'm not saying it will happen, just that it could. Rather than spending Sunday in church praying it will be OK, get the spanners out and sort it. If you do and send me pictures over the weekend I'll try to give you some guidance.
If it was me I'd be phoning the company in the UK and asking them to make a contribution towards the cost of the works, if they re-sealed them recently they must have made a pretty poor job of doing it.
 
Hmmmm......... The point of the little "bullet" inside the rear caliper rests inside a matching depression in a steel actuating block. On the other side of this block is another depression into which the point of the hydraulic piston rests. When the foot brake is depressed, the hydraulic piston "pointy bit" presses against the steel actuating block which moves another block of steel which moves a sharp pawl. When the second block of steel is pushed, it presses against the back of the device which presses against the brake pad which stops the car. The pawl rests on a serrated edge of the bit that pushes against the pad. When the first actuating block is depressed, it is that action which causes the handbrake to be well adjusted.
When you pull on the handbrake, the little "pointy bullet" is pressed against the above-mentioned actuating block which presses the pad against the disc and also adjusts the handbrake mechanism keeping it tight.

One reason for the handbrake to lose it's effectiveness is that the pawl and the serrated edge of the push-rod become worn and the pawl can't get a grip. The result is that you have hydraulic brake but the handbrake can't get adjusted.

I have had the experience with only one rear caliper working but that was caused by a seal leak on the one hydraulic piston.

As mentioned earlier, other causes of one side not working could be that the hydraulic piston has been inserted backwards and the little pointy bit can't push on the actuating block or the rubber seal is installed backwards so the fluid squeezes by.

Eric
 
A very good description of its workings
no wonder I had brain ache after working out how this little lot worked together :p
 
Until I took mine apart I'd assumed the piston you can see pushing on the pad was the hydraulic piston.
Did it get any design awards ?
 
Not being able to take back to UK to place that did the seals, pipes etc - I am having to hand her over to a brake specialist here. They had a look at the rear left last week and will on Friday take the rear right caliper apart.
 
I thought the left rear was the cause of the problem, and was still not working AFTER it was looked at last week, so why is the right rear now being removed, not the left or both. Just because they're a brake specialist, don't for a moment think it automatically means they are competant to remove overhaul and re-fit P6 rear calipers.
 
Left rear is not working (or very little) the only thing we can think of is that there is a problem with the right rear (as the fluid goes in there first or the newly fitted pipes between).

Both were overhauled in the UK in February and whilst the handbrake works fine - the rear left isnt functioning on pedal (after failing MOT 2 weeks ago - had the rear left looked at again and new seals put in (to replace the new seals!) and replaced the rusty large "piston".

As the left brake still fails and hasnt made any improvement - all we can think of is a problem with fluid pressure from the right to the left.

As it is - I dont have the facilities to do the job myself - I cant take the car back to the UK where the job was done in the first place. All I am allowed to do is drive between home and registered garage or MOT station (as my 14 days to get her fixed has now passed)

The specialist I am taking her to here has restored several P6's and numerous other UK classics (restoration specialist) - If he cant fix it - then no-one can (certainly this side of the channel!)
 
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