Rebuilding Calipers- Quick Question

[Tony'z911]

Ok, I started to rebuild my calipers last night. The left rear was my first victim. With out too many problems I was able to get the pistons out, clean them up, as well as the bores and groves in the calipers. I then put a small amount of fresh brake fluid on the small square seal and placed it down in the grove in the boor. I also coated the piston w/ fluid and then tried to push it back in the bore. It only went in about 1/2" or so and is now stuck. I have gotten to this stage with both pistons and they will not go all the way into the bore. I tried to pop them back out w/ air pressure and I can feel the air blowing past one of the pistons. Now the pistons are stuck, they won't go in and they won't come out. Not to mention that one of them is not lined up properly at 20 deg. I have read through a few posts on the board her and the tech article on rebuilding calipers, but everything seems to be very brief on how to re-install the pistons into the fresh seals.

Can anyone please explain these steps in detail and/or offer me any advise.

Well, so much for quick hugh?
Thanks in advance for any info. on this topic.
-Tony

[Pillow]

Hmmm, well I was putting together 944 front calipers last night at about 12:30 slightly drunk and had little problem doing it

Not joking. The package from Pelican was on the front door step and I just had to finish them up. Nice German ATE seals too!

The prep was the long part, 3 hours or so! Then putting them in took a whooping 10 minutes.

For mine the pistons slid in about half way and then tightened up a bit, not sure if the lubrication was claened off too much by the inner O ring and so stopped going. Granted I lubricated the whole bore so not sure. Both seemed identical in the slipping in motion so I consider it the nature of the beast.

For a solution, just clamp them in. Very carefully making sure that the piston is alligned properly before any force is applied. Very slowly start the clamp and the piston should slowly slide in easily w/o too much effort. If there is hard resistence then something is wrong.

... It troubles me that air can pass through the piston... I have a feeling the piston is not straight in the caliper bore.

[Clark Griswald]

I wouldn't press them in any further. The air escaping says there is a problem.

It may be that the seal is not seated squarely in the bore.

Pull the pistons out and have a look.

You might be able to grab the piston with big channel locks and pull. Be sure to use something to protect the sealing surface of the pistons from the channel locks with something heavy (leather?)

[KTL]

To grab the pistons, you can use some slices of wood between the jaws of your pliers. Or you can go to Sears and get some rubber covers that go over the jaws of the pliers. They're the ROBO-GRIP brand covers in a package of 8 or 10 for like two bucks I think.

[Tony'z911]

Thanks for the replies.
I was thinking that I should pull both pistons out to double check the seals anyway. It's funny, I did visually check the one and I felt the other and they both seemed fine. I belive the leak is occuring on the side that I visually checked so maybe the piston is at a slight angle.

Do you think that I could use There is a lip on the end of the piston to hold on the dust cover, do you think I could grab onto this w/ pliers and a rag?
-T

[KTL]

I wouldn't trust the rag. Some pliers (my Channel-Lock type) have some pretty sharp jaws. They rip right thru rags.

Try layering some slices of an old bicycle inner tube. Something that'll not pierce under the pressure of the pliers.

[Early_S_Man]

Tony,

What I recommend for applying pressure to the center of the caliper pistons for installation are the two smallest 'C-clamp' style Vise-Grips ... the smallest works well on 'M' calipers, and the medium one works on 'S' and 'A' calipers.

Since you have the pistons cocked in the bore ... you might as well press on the 'up' side of the piston furthest into the bore, as there is little danger if you get the piston centered again and seated fully into the bore ... it will work fine!

I have been using Dow Corning 4 silicone grease on brake pistons and seals for over 30 years, and VW has been recommending 'brake paste' for that length of time or more, too, rather than just brake fluid! You can use the brake fluid, but the silicone grease eases installation and protects the seals better.

[dhaines]

Just went through this also, for some reason the o ring on the rears are a bit looser and tend to come off center, getting stuck when you push the piston in. I had to take them out several times, putting pressure on center as mentioned above. Some pressure, not a bunch. DO NOT force them. Tried that and all you get is an o ring with a chunk taken out of it. If I were doing it again I would buy some of the silicon mentioned above as it might be slipperier than just brake fluid.

To take it out, clamp one in place and turn up the air pressure. Just watch those finger as it comes flying out!!! If you can grip, great, but on my small calipers there just was no room. Last resort I would hook them back up to the brake line and pump them out. Messy though!

Good luck.

[Pillow]

Also inspect the piston for snags or rough spots, mebbe that is what it is hanging on?

The 944 calipers I was working on were fairly old and nasty and needed a little bit of a polish to clean them up. I lightly wet sanded the pistons with 600 or 800 grit emory paper. Very lightly and evenly otherwise you mess up the bore clearence.

Just a tought.

Again it took a while of prep to get them ready to reassemble the right way. Sure you could do the whole thing in like half an hour but it would be a cruddy job unless the caliper was perfect to begin with.

[BK911]

With out too many problems I was able to get the pistons out


Tony!

You can tell the truth. I'm sure people will get a little chuckle out of it!

BK

[Tony'z911]

Thanks again for all of your info.
I was able to free up both pistons last night. Then after wiping off the brake fluid, I coated them w/ some assembly grease and they slid in w/o too much effort. Then I went to put on the dust covers and they don't seem to want to stay down. They are firmly attached to the top of the piston resting in that groove, but the bottom of them will not stay on the caliper itself to create a nice seal.
I have not put anything on the rubber cover, it just doesn't seem like there is anything for the little metal ring to grab on to. I moved on and rebuilt the other rear caliper w/o any problems until I was putting on the dust covers. I see that the metal ring inside of the rubber can expand slightly, but I don't see what this is for. I had to pry off the old ones w/ a small screw driver but yet all 4 of the new ones are just sitting there and won't attach to the caliper.
Any advise here?
-T

[KTL]

When I was trying to get the pistons out of my fronts, the inner pistons just wouldn't go. I kept having to "massage" them out by clamping the piston back in and pushing it back out with the compressor air. In and out, in and out, in and out. While increasing air pressure each time and each time the piston would come out a little further. Eventually at about 75psi the stupid thing EXPLODED out of the bore. Kinda startles you. Safety glasses are recommended if you've got sticky pistons like mine were.

I haven't done the rears yet (soon this month) but I remember Clark/Chuck saying that you have to put the Kung-Fu grip on the dust covers to snap them into place on the caliper body. Had to use his hands because tests on the old boots would rip them with tools, clamps and the like.