Brake piston is tight

[wwheeler]

Discovered one of my front wheels was harder to rotate than normal. I eliminated the wheel bearing adjustment as too tight, and in the process found that the inner brake pad was very tight and hard to remove. I was going to push the pistons back a little before reinstalling the pads, and the inner piston wouldn't move much but the outer one moved OK.

I have replaced the brake hoses two years ago along with a fluid flush. The car wasn't driven much by the PO, so I wonder if the piston is somewhat corroded. If so, can I attempt to move the piston back and forth to free it up with any success? If I have to dive into rebuilding the caliper, I will go ahead and restore the entire brake system which I am hoping to put off. 

Thank you for the advice!

[ja17]

Hello Wallace,

Exercise the caliper . If  you can get the piston to go back, run it in and out a few times. Make sure you have something like an old worn pad in place so that the piston does not come out all together when you are pushing on the brakes. It should get easier each cycle. If the internals are corroded, the caliper may begin to leak fluid. Hopefully not.   

Pushing a stuck piston back,  can take a lot of effort, I have had to use a large pry bar at times. If all out fails a lot of local autoparts stores still sell rebuilt calipers fairly inexpensive (exchange). There is an early version caliper (early 230SLs and sedans)  they are scarce and will be more difficult to find and more expensive to deal with.
 If you have the early calipers, the best coarse may to swictch to the more common later units. They bolt right up.  Pads, repair parts and complete replacements will be much easier to find and much less expensive in the future.

[graphic66]

I have found that it is the pad itself that isn't moving within the caliper sometimes. I now "fit" the pads when installing them by taking a file and cleaning the edges of the metal backer up until the pads move freely inside the caliper. I had some brake squeel after installing new pads and I removed them and cleaned them up and put a very small amount of never sieze on the edges and the squeel stopped. Now I always fit the pads to the caliper. The pads should drop right in and not bind up on the caliper. The backing plates are stamped out and have rough edges when new. This may be your problem. Of course it could also be a bound piston or a plugged hose or passagway in the caliper. But, you wouldn't be wasting your time "fitting" the pads. You can also apply some anti squeel paste to the pads while they are out. That would't hurt anything.
  And remember if you have Ate calipers DO NOT split them in half, not sure why but the BBB warns against this. The Girlings can be split. Also when replacing the pistons put the rubber seal on the caliper and then push the piston into the caliper or you will have a very tough time getting the seal on after the piston is in.
 You may just want to rebuild the calipers, it is very easy to do and very inexpensive. I know that some mechanics wont do this because of liability and reputation issues, but if it is your car and the calipers are in good shape it may work fine for you.

[xcashewx]

my pistons were corroded in place. i couldnt get the pads out at first, and one wheel had a lot of resistance.  I took the calipers to a local shop and they pulled the pistons for free. just cleaned them up and put new seals in and they work perfect.
about 30 minutes or so of work and the only thing that was paid for was the seal kit.

[wwheeler]

Thanks for the replies. My calipers are the newer ATE.

I will try exercising the pistons first as that is the easiest and least expensive. Exercising seems like a common sense approach but it is good to hear it from experience because you don't want to make anything worse than it already is.

I did notice on the other side, the pads were tight on the sides as mentioned in the above post. Next time I have them out, I will fit them with a slight clearance. Good tip! This side with the problem however, the pads were loose enough on the side fit. If all else fails, I'll get a rebuild since the bleeder fitting is broken off of that side!

[wwheeler]

Success with the piston exercising!!!!

When I first tried to move the inner piston into the caliper , it was very tight but it moved a very little bit. I then placed a thin piece of wood between the rotor and piston and applied the brake. The piston moved outwardly to meet the wood. Pushing the piston back now was a little easier. After repeating the process about 10 times, the piston was much easier to move, almost by hand pressure.

Now the wheel spins easily after the brake has been applied and released. I would think that it would be good maintenance to check ease of tire rotation every year on an infrequently driven car to find a problem such as this.

[Bonnyboy]

I have started taking my brakes apart - drivers front because its the easiest.  Everything came apart easily except I had a heck of a time getting the pistons out with air.  I can get one out but then there is no pressure to get the other one out, so I clean it and put it back in and hold it in and pop the other one out and clean it and put it back in and rotate that sequence until they were both able to come out with air and two pry bars on opposite sides and air helping it out. 

With the bleed screw and the brakeline holes open and the two holes in the piston cavity open there isn't much incentive for the air to push out the remaining piston. 

Is there a trick to removing both pistons or is it a one at a time sort of thing.  I was thinking of placing something in the vacated hole that would slow down the escaping air (wood dowel with innertube wrapped around it) secured by a welding vice grip so I don't get my fingers squished when I apply air.

Ian

[Benz Dr.]

I use a large C clamp to move sticky pistons. If the wheel won't turn freely by hand, you have brake problems. It's one of the first things I check before I remove any wheels. Front end play, tie rods, drag link, idler arm bushing, king pins and wheel bearings along with dragging brakes.

I often find worn out idler arm bushings, king pins, and loose wheel bearings. Push on the top of your tire while pulling out on the bottom. There should be no play. If there is, grease the king pin bushings and try again. If the play is gone the king pins are worn. If the play is still there it's likely loose wheel bearings. Upper and lower pivots can be worn but all the spring tension needs to be removed from the suspension before you can test for loose parts. Raise either front wheel with a floor jack under the lower control arm and this will allow free movement if anything is loose.

[Bonnyboy]

OK - this morning before the birds were awake I got the other front brake caliper off the car - popped out one piston with the air hose, pulled out the seal and dust cover, slipped the piston in a rubber glove and inserted it back in the bore, held it with a vice grip and blew air into the bleed hole and the other piston came right out.  Putting the piston in a rubber glove gave just the right amount of interference to pop the other piston out.

NOW THAT WAS EASY.

Ian