New brake pads too tight

[911-ST]

Hello,

I have got new brake pads to fit on my front brakes. But I have a problem to fit it in. The space between the piston & the disk is too tight on one side. I notice that on this side when I push back the piston, as soon as I release the pressure it comes out again just a bit. But that prevent me from putting the pad in place. I have tried the same procedure with the bleeding valve open by I end up with the same result : the piston comes back out 2 mm after the pressure is released.
Does anyone have a clue how I can solve that ?

[DRACO A5OG]

Place a 2x4 block of wood on the brake pedal and press in about 2 inches by pushing the seat up against it.

Place a tube and catch bottle on the valve and open the valve. Let the fluid flow out but insure it will not back flow air into the system. The block of wood will help prevent this from happening.

Push back the pistons, plop the new pads in, tighten the valve. Make sure no air goes into the calipers. ( make sure the pad's path is clean, use a wire brush and brake parts cleaner. )

[Jcslocum]

How old are the calipers and have they ever been rebuilt?? It might just be seal hysteresis but it could be gunk buildup behind the piston seal pushing it back. Make sure they turn easily and smoothly once back together. If they are dragging, more than just a wee bit, that's not good.

[Quicksilver]

The seals are pulling them back. Compress the piston and while it is clamped back rotate the piston a touch to get the seal to "relax". I use some heavy duty snap ring pliers to turn the pistons to the correct the alignment when doing a brake job. That should work for you.

If you have a problem with excessive seal retraction replacement with ATE brand seals can help. (A lot of the others seam to have excessive retraction.)

[911-ST]

I read your post with attention I was sure that the rotation trick was what I needed.
But no luck .

The brake system is new or look like new.

I also notice that the piston brake was doing that "pull back" sort of thing even when it was 1 cm "out" with the seal partially removed (if we speak about the one which is visible).

There is behind the brake pad a sort of "rubber coated plate" can I remove it ? I might get back the 1mm that I need.

[NICE 69 S]

There is a lot of good advice above, and the pads should fit without alteration. If all else fails, I would sand down the face of the brake pad (friction material) before I would take the pad off the back. It keeps the brakes from squeaking. Depending on the year, these brakes may have a pin in the caliper that goes into the back of the piston that is there to keep the pads in contact with the rotor at all times to keep the pedal high. This sounds like what you are describing. When you let your foot off the brake, it keeps the pad against the rotor, and also keeps a slightly warped rotor from pushing the pads away from it.
See here, you can see the pin and receptor in the piston about half way down the first page::

http://www.early911sregistry.org/forums/showthread.php?44148-Vintage-911S-Caliper-Restoration-Thread&highlight=caliper+rebuild
Bob B

[porsche930dude]

most mechanics i know almost always remove the backing plates. they seem to cause more problems than they solve

[rusnak]

Were your calipers off the car recently? Is the rotor centered in the caliper, or offset to one side.

I would try to find out the cause and fix it, rather than force fit or modify parts. You might also take the brake pistons out with the caliper off the car, replace with original Ate seals, and even try Ate caliper grease.

[sugarwood]

Did you remove the lid from the brake fluid reservoir?

[rusnak]

Hopefully it doesn't have a Mityvac on it pumped up to 12 psi.....

[pete3799]

You sure you have the right pads for your calipers?

[Jdub]

The pushback is natural. You need to vent this closed fluid system by bleeding off some of the fluid. In the simplest way, you have too much fluid in the system for the new, larger size of the pads you are installing. So comments like removing the MC cap, opening the nipple, etc. all mean you should be "venting" the fluid if you ever wish to find the larger amount of room you need.

When you finally bleed the system and take it for a careful drive with a few pressure stops, it will re-center the pucks and whatnot and you will be in better shape. Let us know.

[bkreigsr]

Originally Posted by porsche930dude

most mechanics i know almost always remove the backing plates. they seem to cause more problems than they solve

huh?
the brake pad backing plate or the dust shield attached to the hub?
Bill K

[911 Rod]

Do you have the right pads?
I had the same problem and it turned out I had the wrong pads.
The one side would go in really tight and the other would not.

[911-ST]

I am questioning the pad size there seams to be two size for the thickness 185 & 150.
I have now the following pads 91135194006 20011 185 164 which I believe is 185 mm thickness.

I got an older invoice where it mention 91135195009 but I don't know what is the supposed thickness of this pad.

I purchased the car recently with these pads on the only detail I have on top of that is that I am supposed to have the Alloy S Caliper.

[911pcars]

There's no check valve in the brake system, just gravity. Thus, bleeding off brake fluid is just an exercise in refilling the reservoir (unless following Draco's [Jim's] method with a tube). As many suggest, it's probably a case of aggressive piston seals.

S and A pads have the same silhouette, but S pads are thicker by a bunch. That's a metric phrase as well.

Sherwood

[NICE 69 S]

If your car is supposed to have the "S" caliper and doesn't, and the "S" caliper pads (presumably the one's you have) are much thicker, there is your problem. Find the pads that go with the calipers you have and that should solve your problem. I would also measure the thickness of the rotor to be sure it is the proper one for your car.
Bob B

[kodioneill]

Wrong pads.

[911-ST]

I solved my problem by removing 2mm of the pad using sandpaper. I am not happy with the solution but it worked and it was easy to do.

[Jdub]

I have to agree mis-matched or wrong parts. You mention the system "looks new" so I'd take an inventory of parts you have and be sure this is correct. This is not an area to mess around with.