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MG MGA - Brake pad drag - Rotor changed colour!

First of all. God daaammit! fdf$$#%%%).

After deciding that the brake system was bled enough (until I get the Eezibled) to take it for a quick spin, I did.

As the brake pads (caliper) were new, in the moss instructions, to do the bed-in procedure. So I did several brake sequences without coming to a stop.
I stopped to look at the brakes and they were looking fine, and the discs were hot - normal.
The I drove another 5miles, and when I stopped again to look at the discs, the front right rotor was had slightly changed color (light brown). SH*****T. Obviously getting too hot. Did I say SH****T.

I should have stopped cos I heard a light cyclic rubbing of the wheel at some point.

When do home I took the wheel off, and the brown colour had rubbed off the area where the pad rubs.

Now, here is the problem. I remember that when I put in the outer pad in the caliper it was a tight fit due to the rubber anti-squeal shim that the pad has. I though the rubber would get compressed and seat itself properly. The inside the pad went is as a loose fit.

So the outer pad rubs, and drags slightly as the wheel spins, probably enough to heat up the rotor to that temperature.
I immediately checked if the rotor had warped, but it seems to run true. As least as true as when I installed it.
Do you think the rotor is destroyed now? would the heat have changed the properties of the metal?

The pad goes in well without the shim, but it is a tight fit with is. What do you suggest I do?

a. Take the caliper off, and push the piston further in? I wonder if I do that, will the piston return to where it is now when I press the pedal and be back to square one? Could the piston be malfunctioning and not returning properly as it should?

b. take of the shim and wait until the pad wears slightly, then put the shim back on?

I tried switching over pads but the same happens with the other.

Your ideas and advice are appreciated.

I am using the Ceramic premium pads from Moss.

BTW - the car brakes great now. The I can feel the front disk biting now good, as the front of the car dips slightly when I brake hard. This did not happen before.

Pretty p***d-off.

Gonzalo Ramos

The front rotors can get very hot. From your description I don't think you have anything to worry about. When I drive over the mountains here, after a quick descent the font brakes will smoke noticeably if I stop and there is a pretty good smell too! I think you can get the rotors to pretty much glow without risk of warping.
Neil McGurk

Sounds like the calipers are not centred over the rotor. Not sure how that can be except a bent caliper holder or dirt on one or other mating surfaces.
Art Pearse

Brown is OK, Blue is not.
You say that the brown rubs off where the pads are rubbing, so that is probably oxidation. If you don't feel any pulsation on the brakes as the pads rub, you don't have a problem.
If you can further compress the piston into the caliper, do it. It won't hurt it, but watch your master cylinder for overflow. Also, before moving the car, apply the brakes until firm. You don't want to move the car with slack in the brakes.(Voice of experience.)
When you brake, the pistons will come out to stop you but can't go any further than they already are. If you have room for the anti-squeal pads, leave them in. If you don't, then you are better to take them off for a few hundred miles and re-install them later, but they should fit fine.
Good luck.
Mike Parker

Ok, thank. Your comment re-assure me.
I guess the damage is not that serious then...
I will mess around this weekend trying to push the piston back in, see what happens.

Thanks guys!
Gonzalo Ramos

Gonzalo, you should be able to push the piston back in without removing the calliper, lever against the rotor with a pry bar/block of wood or anything that fits! I changed the seals in my callipers and couldn't get the pads in even though I thought I had pushed the pistons right in. When I put a long pry bar between piston and rotor I got another 1/8 inch or so (without using excessive force) and the pads dropped straight in.

X2 on what Mike says about pumping the brake pedal before driving!
Neil McGurk

I did as suggested and pushed the piston right in, so the pad dropped right in. I didn't want to score the new rotor so I did not use the McGurk technique, having take of the caliper.

Anyway, when I put it back together I realised that as soon as I pumped the brake a couple of times with the pads in, the outer pad would bind again (one the pedal was released) on the rotor causing a very big drag, i.e. not releasing.
The inside brake pad as a snug fit, but released and 'floats' in the slot (as it should).
Also the other wheel rotates freely after releasing so no problem with the MC.

My conclusion is that the out piston is not returning as it should (semi-seized?), so I will rebuild it. Luckily I have the rebuild kit already.
Do you agree with this conclusion? Has this happened to you before?

What a lovely task for the weekend. I will have nightmares tonight about brake fluid spillage on the floor of the garage. 8-)

Gonzalo Ramos

Does the piston seal have a right and wrong way to fit?
Art Pearse

Gonzalo, I would say your diagnosis sounds correct! And my method didn't score the rotors! LOL
Neil McGurk

Gonzalo, don't have nightmares about brake fluid on the garage floor, have them about brake fluid on the paint! (just kidding, watch out and clean it up quickly.)
Mike Parker

Brake fluid washes off with plain water, from paint or floor.

Fletcher R Millmore

I believe the outer seal of the piston does not have a right and wrong way... Mine has a 'U' shape cross-section, and I think it must have 'grown' cos the diameter is slightly larger than the piston OD. You need to push it from several side to get it to fit all around and not pop out on one side.

Gonzalo Ramos

Your problem is shared by many others-----
It is caused by not changing your brake fluid often enough and this lets rust and crud build up in the caliper. - Then ,when pushing the piston back to fit new pads the piston binds up on the junk in there and gives problems ,sticking and not releasing properly.
I would suggest you reco both calipers to get even performance and don't forget to change your fluid regularly. Also NEVER lever on the rotors to push the pistons back
The 'U' section seals you speak of are only the dust seals, The actual fluid seals are a square section 'O' ring in a groove further into the caliper. This seal is square and can be fitted either way round - It is the groove in the caliper which is machined with a slight taper to give the O ring the correct attack angle to seal against the piston.
Really check for imperfections on the surface of the pistons - If there are any rust pit marks get new pistons. Every pin hole here has the same effect as a pin hole in a hose - It will leak fluid.
Use plenty of rubber grease during assembly of your calipers-
Cheers Willy
P.S. I'm sure your rotor will be fine---

By the time you have been through all of this so many times, new calipers at $120 each seem very reasonably priced indeed in comparison
dominic clancy

Im at a loss.
I spent most of the evening rebuilding the caliper and putting in new seals. Messy!
I changed both of the pistons seal and dust seals. The DPO fitted the retainer the wrong way round so they did not retain anything, and the seals kept falling out.

Anyway, I put everything back together and I still get the same result. The inner piston retract as it should when you release the pressure, but the outer stays out jamming the brake pad onto the disk.

Since the seals are new, I don't know why it would not retract by elasticity of the seal.
Maybe the centre pin is jamming?
Any ideas for my tomorrow morning session will be highly appreciated!

I will take it out again tomorrow to clean the inside of the cylinder thoroughly and see if I can find something else wrong.
The pistons are still of the original type in this caliper (the other side has MGB pistons), but are generally in OK condition, with some very minor pitting but just close to the outer edge, not where the seal is in contact with the piston.

BTW the beautiful red paintjob I had done on the caliper has turned into a sticky goo! ;( Will have to do it again.

P.S. Dominic, I agree with you about buying a new one, but I took it as challenge to successfully rebuild it to leave it as original as possible.
Gonzalo Ramos

Gonzalo Did you split the caliper halves apart when you rebuilt? I did once on a sticky caliper and found a piece of rubber seal caught in the transfer hole between the two halves, could be your reason why one half does not release. Chuck
Charles O'Brien


I assume that the centre pin that you refer to is the very badly named retractor pin.
The first thing you should do is remove the centre pin and throw it as far as you can. They cause endless jamming of the piston on the Dunlop disc brakes on the Jaguar E-Type. They serve no purpose and were deleted from later production disc brakes.
I assume that your MGB pistons on the other side do not have the pins?

M F Anderson

Dominic is right!!??
Satisfaction saying "I did it my self" priceles. Your daily driver brakes are the same, or the priciple is, just think how much you save to fix them and your helpers yourself.

m zazvorka

In the Twin Cam Dunlop disc brakes the centre pin is called the retractor pin and with the 1600 brakes it is called the guide post. By either name it is an unnecessary item.

M F Anderson

I did not split the caliper because I did not consider it necessary.
As suggested I will try to take out the center pin and clean well the bore see if that does it...
I going to work on it. News about results later.
Gonzalo Ramos

Spent about 3h messing around with it again.
I took the piston out, and changed the seals again, cleaned the bore, and cut off the guide post off flush.

The result is the same. No noticeable improvement.
I don't understand what is happening.

The only thing left I can think of is splitting the caliper to see if the is a block in the channel which would not allow the release of the pressure.
I tested it with air under pressure and there does not seem to be a block in the channel.

Only other thing would be to replace the piston for an MGB one, but I don't think that would make a difference given that the OD is the same.

What else would cause the piston to not retract? The seal is in there and it is springy, I can feel it when I push the piston....

It is noticeable that when the pressure is release the inner piston retracts about 1mm, but the outer does not at all.

I am hoping maybe the seal will get softer with time and the brake fluid and improve?

Further ideas welcome. I am at a work stoppage condition! ; 0

Gonzalo Ramos

Gonzalo. The only thing that retracts the pads is the actual motion of the disc. It's flexing while in motion is sufficient to clear the pads.

Gonzalo, it is also strange that the one piston retracts as much as 1 mm. It should not! Is the sticky piston actually a free fit in the bore? Sorry, had to ask.
Art Pearse

If it is the flex of the rotor that makes the pad clear then maybe I am worrying about nothing.
Does the rotor actually flex?

The 'sticky piston' is a free fit in the bore with all seals removed. I guess that is your question Art.

When you look through the side the is a small line of light (gap) between the pad and the rotor. Maybe it is less than 1mm, but can see it. The pad is loose and can be take out easily.
Whereas in the other side I cannot see this gap and the pad is a tight fit on the rotor.
I can still spin the wheel but there is some drag...more than in the other wheel.

I am puzzled because they worked fine before I rebuild it! :S
Gonzalo Ramos

The comment that it is the flexing of the rotor that moves the pads back from being in contact is incorrect. The pads are retracted by the seals returning to their natural shape. When the pads move forward on to the rotors the seals are distorted, but do not move along the piston. When hydraulic pressure is released they return to their normal shape and retract the pads. The seals only move on the piston when needed to compensate for wear (the pad movement exceeds the seal limit to deform).
It sounds like one piston is retaining hydraulic pressure. Have you tried opening that one bleeder nipple while it is pressing on the rotor? (no pedal pressure of course).

M F Anderson

Yes that is the same understanding I had, which is details on Barney's site.

I have not tried opening the nipple to see if it releases but will try tomorrow.
If it does release I guess that the passage from one side of the caliper to the other is clogged, and the caliper needs splitting.

From reading Barney's site the other hypothesis I was coming up is that the seals installed are too tight or stiff...
Gonzalo Ramos

If the pressure does release on opening the one nipple then you do have a hydraulic blockage on one side.
Before splitting the caliper remove the hydraulic connection and blow air through to see if the cylinder assembly is blocked somewhere. You may be able to clear it without splitting.
Do both pistons have the same clearance in their cylinder bores? This clearance affects correct seal deforming. Are the seals in both sides identical and both of the same age (as far as you know)?
If you replaced the seals they should both be the same in operation (Barney's comment on seals being too stiff). One seal only should not be too stiff.

M F Anderson

Gonzalo, if you can spin the wheel you probably don't have a problem! Check for pressure as Mick describes, if there is none, drive the car. It may just need bedding in.
Neil McGurk


In general I agree with your comment "Satisfaction saying "I did it my self is priceless."

But with brakes, the hassle and savings of messing around are not worth it, especially for such critical components. I'll replace seals in the MC because it is so easy to do. Anything else in the brakes gets replaced when worn out or broken or at the end of a normal service life whether broken or not. Easier in the long run and much safer too.
dominic clancy

Seals on both sides are new. Just changed them. So I think the clearance should be the same.

The inner piston still has the guide post. I hope that is not making the difference.

Neil, you may be right, I may just have to bed it in. The pads had the rubber covered shims which are springy.
I will see what the further diagnostic is tonight...
Gonzalo Ramos

"... rubber covered shims..."????

Never seen that and absolutely not as it was designed!

Sounds like you found the cause of the problem!
Neil McGurk

That is how they come from Moss.
It is a thin piece of metal with thin film of rubber over (which in fact starts to get damaged as soon as the piston pressed it a couple of times).

Could be the problem but that would cause an issue on the other pistons, and it doesn't.
Gonzalo Ramos

Well, after another session in my improvised workshop, this is what I found.

After pressurizing and releasing, if I open the bleed nipple there doesn't seem to be any residual pressure. And the piston does not retract any more as a result of opening the nipple. So ruled out a blocked internal channel in the caliper.

Being curious, I check the left wheel and in fact it also drags am much as the right one!
I can turn either of the wheels easily with one hand, but I can hear the pad rubbing.

I am coming to the conclusion that this is probably as it should be...I guess it is a feature of the MGA and the pad will always drag a bit.
Also I think it still needs some bedding-in as I removed the pads and pistons several times.

I think Neil might be right, if I can spin the wheel maybe I should just drive it! and maybe I have been worrying about nothing. When the rotor got that hot I was pushing the brakes quite hard and often for the bed-in. I would not use the brakes that much in normal conditions.
Gonzalo Ramos

It's time to enjoy driving it. (But in the back of your mind you will be thinking of the brakes.) Just check for excessive heat ocasionally when you think of it. We all have something on our cars that we think could be better and it gives us something to think about as we enjoy the driving experience. We are always listening for the odd noise, or slight vibration that could mean that something is about to give us trouble. It keeps us on our toes and alert while driving these old fun cars. If you wanted a care free driving experience you would have bought a Miata.
Ed Bell

I think every car with disc brakes will rub to some degree if you apply the brakes and then spin the wheel by hand. The key is the spinning. If the wheel spins with no more inertia than its own weight, then clearly the brakes, although touching enough to be audible are not having significant effect and certainly not enough to be perceivable or even get warm on a moving vehicle!

Your job is done! drive the car!
Neil McGurk

Yep, if I spin the wheel with my hand it will spin one turn or two and then stop. I guess I am in the clear then! woooho.
I need to finish bleeding it now and take it for a good ride!
Thank you guys for the support. I really appreciate it!

Gonzalo Ramos

I've followed with interest Gonzalo's tribulations in getting his MGA serviceable and more than once its caused me to look again at my own car. This time its the dragging brakes. NM, are you saying that it should be possible to give front wheels with disc brakes at least one rev spin by hand after applying the brakes? I get nothing near this -perhaps a 1/3 rev at most. In this thread I've not seen any mention of the one way collapsible valve cup in the brake cylinder that keeps 8 psi in the system in order to stop the caliper pistons from pulling back too far. 8 psi will potentially give a force of 25lb on the discs that may be reduced somewhat by the counter action of the seals. Because of this I always thought that brake drag is something owners have to live with but I'm interested if someone has managed to tweak the system to avoid it. I heard stories about leaving out the valve but never felt this wise to do.
J H Cole

With the car on jacks press the pedal and check the wheel by spinning.You will hear a slight drag. Then hit the tire with a big hammer and then spin the wheel. you should notice the difference in sound. This is what happens when you are driving. The bu,ps and lumps the wheel receives moves the pads back a little bit.

John, that was not my meaning, the opposite in fact, but it is a very good and relevant point. What I meant was that if the wheel turns easily even with a slight dragging then there isn't a brakes locking on type problem (that Gonzalo is concerned about).

I just did a check on my car and the wheels will spin a turn or two, but only because I washed the car before putting it away so there is a slight oxidation of the discs. If I spin the wheel straight after a run it will turn much further.

Then I gave the brake pedal a good hard push and the wheel was as you describe, about 1/3 of a turn on the offside and I could get a full turn on the nearside. That confirms that there is residual pressure.

Not sure how to confirm the figures though!
Neil McGurk

I am glad my thread is useful to others!
I general I would not say the non-return valve would hold 8psi. The aim is that it hold the pressure which you are 'double' pumping, but the pressure should release completely as soon as you release the brake pedal.
If it does not, the you may have an issue in the MC. Check Barneys site, everything is detailed there.

I agree with you Sandy, I think that once the car is moving, vibration and movement of the disk + air will cause the pads to open slightly more.

Tonight I am going to finish bleeding the car and take it for a ride. Need to get it running good cos I have a drive down to the Keys (Isla Morada) with the British car club of Miami, so want to make sure everything is good!
I am also working on tuning the carburation after a full carb rebuild, but that will be a new thread.

BTW any of you guys go to Key West last weekend for the gathering? I missed it but may have to join next year!
Gonzalo Ramos

Neil ok I've re-read your post and I see what you mean now. The 25lb is simply found by multiplying the residual 8 psi pressure in the brake line by the face area of the caliper piston - about 2 inch diameter. Like you I find that the longer the time the wheels have after braking then the more free they become. I did a check on my master cylinder by unscrewing the end plate and allowing the pistons to move back against the push rods to obviously clear the compensating brake orifice - there was no change in brake drag. If there is a culprit here it might be non return valve that is too stiff and not collapsing soon enough once the brakes are released. I haven't got the equipment but it would be quite interesting to try and measure the residual pressure and compare it with the 8 psi. Gonzalo, yes I thought 8 psi sounded a little high but its in the workshop manual page M6 for me.
J H Cole

This thread was discussed between 22/04/2010 and 28/04/2010

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