What causes sticking calipers?

Post Wed Jul 30, 2014 11:18 am

18 months ago my rear right caliper seized up, so I figured it was a good time to refresh the brakes. I put all new brake hoses in, redid the hard lines all the way to the master cylinder, and rebuilt the caliper. Didn't help - the caliper started sticking again.

So I replaced it with another (used) one which I also rebuilt before fitting. That one ran for about a year, but has now stuck again to the point where I can't even get the piston out (really, not even with molegrips).

I've had to order another caliper in now, but I'm wondering what would cause this sort of failure rate? Am I just unlucky?

Post Wed Jul 30, 2014 1:25 pm

What's sticking, the piston or the sliders?

Post Wed Jul 30, 2014 1:39 pm

Usually caused if you've left the car standing for months, or if there's water in the brake fluid (it's hygroscopic, and absorbs moisture from the air).

You could try using a pry bar or G-clamp to lever the piston back in and try to work it in and out.

Post Wed Jul 30, 2014 2:26 pm

The sliders are fine, but the piston is jammed solid. A crowbar can't get enough purchase to shift it, and tapping it back in with a hammer just jammed it in further.

I'll get some butane for my pen torch to burn the rubber seal out, but I still don't understand why it got stuck in the first place, especially with the car in almost daily usage. Maybe I just need to change the fluid more frequently.

Post Wed Jul 30, 2014 2:32 pm

Never come across a seized one on an E30, but the only way I can see that it's possible is for water to get past the 'dust' seal and corrode between the piston wall and the bore.

Post Wed Jul 30, 2014 2:45 pm

Lack of use is the usual culprit,allied to water in the fluid.

Possibly being a little more 'enthusiastic' with the pedal may help?

I have not had any problems with the cabbie brakes sticking,and that can stand for weeks between trips out.Handbrake shoes sticking,yes,but not calipers.

Post Wed Jul 30, 2014 7:06 pm

What fluid are you using? The 'racing' type fluid absorbs water quicker than other fluid, ive had problems with it after leaving cars standing over winter myself
Dot 4 is what I use now

Post Thu Jul 31, 2014 6:13 am

Hook a european (10mm fittings) grease gun up and pump it out. 6000psi. Dont use air 150psi.

Post Thu Jul 31, 2014 10:08 am

We'll see how it goes. When I removed the boot the piston had a sheen of rust on it, so i'm guessing moisture was able to get past the boot somehow. Only other thing I can imagine is low-quality rubber that's somehow gone rock solid; that's the only thing holding the piston in place at this point.

Post Thu Jul 31, 2014 12:50 pm

DanThe wrote:What fluid are you using? The 'racing' type fluid absorbs water quicker than other fluid, ive had problems with it after leaving cars standing over winter myself
Dot 4 is what I use now

I always bleed with dot4, 5.1 is reserved for very limited occasions.

For a road car if your boiling fluid then you are either doing it wrong or need to consider upgrading the brakes, road cars don't have class spec limits and dot5.1 is just a stop gap.

In fact the water absorption issue is why I stick with dot4 in the downhill bike brakes as well.

Try using some very high temp grease behind the boot, If it melts a bit then the boot should hold it in.

Post Thu Jul 31, 2014 5:00 pm

I had to machine an "o" ring groove in some components at work so I made a fixture for them to sit in . It had to be a decent sliding fit (80mm dia) and it was . The items went in alright but would not come out without some persuasion which seemed very odd until I hit on the idea that the cuttings were almost like dust and were being trapped between the component and the fixture. A touch of grease to stop the ingress and it came apart first time.
So, dust is quite capable of seizing anything with a good fit involved.

Can grease be used behind the dust covers?

Mart.

Post Thu Jul 31, 2014 6:42 pm

Yes red rubber grease is suitable. Millers, Castrol and others sell it.

Post Thu Jul 31, 2014 7:15 pm

The majority of seized pistons I come across have some form of defect in the boot (or no boot in some cases). Bear in mind that it is not only rust on the piston that causes them to seize, but also rust on the ring of metal that sits between the groove for the boot and the groove for the seal.

However, I did have a Subaru Impreza with some 4 pots on it once. One of the pistons was seized and was an absolute nightmare to get out. Took four of us (one in the car, one to hold something solid between two of the pistons and to control the seized piston coming out, and two to stop the opposing piston from coming out). The weird thing was, the seized piston had no rust on it at all, and neither did the cylinder. It was as if the piston was kicking over (not coming out straight) and jamming up, rather than actually being seized in.