What are the chances of two faulty callipers?

[ya_crow]

My rev4 n/a failed it's MOT the other week on handbrake efficiency and had general poor rear brake efficiency. So no big deal I thought, it'll be a cable or a calliper. I examined the o/s calliper and the handbrake mechanism was seized. A good dose of WD40 and a bit of tapping the mechanism back and forth and it freed it up a bit. It seemed to pull when applying the handbrake at a slow roll and I thought it'd be enough to pass and then I'd strip it all properly and clean both callipers up. Next day, put it on the rollers and it did fukc all. Hardly any braking. So this time one of my colleagues did the same for me ( I work in a VW dealership and he has access to a ramp on Saturday afternoon) again he gets it freed up and working when it's up in the air but by the time the wheel hit the ground there's sod all happening when the handbrake is applied.

Again I think, no major issue, I have two spare rear callipers which were destined for my V6 but I'll chuck them on there and that'll put it right. Whilst I'm at it I ordered new cables and pads and then got carried away and added a set of braided lines to my eBay addiction. Belt and braces fix, nothing left to go wrong? Yeah I thought so. The above £200 shopping list all bolted on, bled and ready for the rollers.

And the difference. Absolutely none, or at least not enough to pass an mot. The near side pulls 200 whatever brakes are measured in. The off side struggles to reach 50 and this is mirrored on both the handbrake and foot brake. My colleagues who know more than I do about anything oily and mechanical are now guessing that I've just got two duff callipers. How likely is that? My rev1 had one calliper replaced in 23 years and 160k miles. I know it's possible but are there any more likely causes.

I hadn't planned on having two money pits when I bought the second MR2 but it's starting to turn out like that.
just in case the mot failure sheet means any more to anyone than it does to me.

[doogal44]

Check the adjuster in the centre of the two cables when you pull the handle to see if it's applying even force to both cables. If it's not pulling evenly you will get the described problem.
Not sure about the foot brake though.

[ya_crow]

I did think of that, but as you say that wouldn't account for the foot brake. I also thought about the possibility of an air bubble trapped in the calliper but that again wouldn't account for the poor handbrake and it's unlikely. Bleeding the brakes took forever too, four attempts before any satisfactory pedal feel. Frustrating how the simple things can cause such issues.

[adamh]

it does not take a great deal to change a caliper. i would swap them out with your other calipers, put it to the rollers, see what result you get. if its identical.. then its not the calipers. whats the condition of the rear discs? how flat are they?

this is bad.




even though i dont suspect its your discs

this is normal scoring and even if the whole marked area is a little deep its nothing to worry about for general driving condition, the pic is showing for condition of disc face only with respect to grooves



one more thing to note, if you added new pads to either (especially if it looks like the top picture) the braking force may be less than normal, because the old pads wear a pattern in that matches the disc. if it failed in the first place without changing anything... its not really likely the disc or condition... the force would still be ok even if they were worn out fairly but within thickness limits of test like the above picture.

[ya_crow]

I've already swapped the callipers Adam, albeit for second hand ones which look in good condition and the mechanisms are free and clean. The discs are okay and the surface is flat, I have another set of discs too as I bought full a set of discs and callipers from someone who was breaking a car. It is possible that the original and the swapped callipers are both in need of refurbishing but having chucked loads of cash at a simple problem already I thought I'd throw it out there for ideas.

[adamh]

I've already swapped the callipers Adam, albeit for second hand ones which look in good condition and the mechanisms are free and clean. The discs are okay and the surface is flat, I have another set of discs too as I bought full a set of discs and callipers from someone who was breaking a car. It is possible that the original and the swapped callipers are both in need of refurbishing but having chucked loads of cash at a simple problem already I thought I'd throw it out there for ideas.

great , object of exercise being i.d the condition of the disks !, it did not sound like them but ya didnt mention them, just crossing out a blank.

edit, btw.. go and get some use into the new pads before you get it into the test place, go drive round a few housing estates carefully, try and bed the pads in a bit more , you can have the best rebuilt brakes in the world, they will still have a reduced braking force until they bed in proper.

slotted and grooved discs and ebc red stuff pads.. (assuming all kit is new) don't share so much the same failures!..they just stop you quick.

old disks and new pads is bad, i used to re-dress my discs on the lathe if i used new pads.

[Goldy]

Did u follow the workshop manual process as you wound the pistons back, I seem to remember something about a specific number of turns to set it correctly otherwise you don't get proper force, but I could be wrong

[thomp1983]

It's unlikely the actual brake part of the caliper has failed on both calipers without obvious corrosion or leaks. The handbrake Is a different matter.

For the rears you need to wind the pistons all the way in then back out a turn and align the grooves with the pin on the back of the pad.

How have you been bleeding them? Of all the brakes I've bled mr2's are the worst I've known for air seeping in through the threads with the bleed nipple open for bleeding. I always put a bit of ptfe tape around the bleed nipple threads and refit it then put a smear of grease around the bleed pipe end when it's on the nipple and around the nipple threads then bleed the conventional 2 man way

[Paul Woods]

Some light entertainment about the subject when i was told how wrong i was from people who didn't know better, makes funny reading 10 years on.

http://www.imoc.co.uk/forums/viewtopic.php?t=6372

[nads1978]

Some light entertainment about the subject when i was told how wrong i was from people who didn't know better, makes funny reading 10 years on.

http://www.imoc.co.uk/forums/viewtopic.php?t=6372

I was going to post this on your link, over on IMOC, but I can't post pictures there, new member, need more posts - can't be arsed.
I'll post it here, it may help someone out. Maybe.

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I know it's an old thread, but it has helped me out (Thanks Paul), So I thought I'd back up Paul & help to illustrate with pictures... We all like pictures right?!
It's on my Mk3, I guess it's similar to the others?

This is the screw, about which the piston will wind in/out. It is locked so it can't rotate. It can push, which is what it does when handbrake is pulled.
You will see that unlike a regular screw thread on a bolt, it has multiple threads running together; meaning it can start (unlike a bolt) in more than 1 position [this is important to understand for later].





I have wound the piston right in & parked it in the finished position.
It is protruding 2.5mm from the casting:




I wind it out 180deg and park it in the finished position.
It is protruding 6.6mm [so we have travel of 4.1mm]:





Lets consider what Paul was saying about clearance of brake pads... What if they now wont fit? Well, panic not. Paul said to wind the piston back to the start position, but 'overshoot' by 1/8 of a turn. As before the start point is 2.5mm:




Lets wind her out, to the final position, 180deg + that 1/8 turn. What has happened here, the piston adjusting mechanism has picked up on a different thread on the main shaft (remember, earlier I said about the threads?). Now we have a protrusion of 5.5mm - so we have travel of 3mm. 1.1mm more room for the pads than the 1st attempt: