740 wagon [1989] - Brake Caliper Mounting Problem -- Rear Right ATE

[awcarvey]

----- Background -----
Prior to this incident, my Volvo 740 was driving fine, rode smoothly, and did not have any steering drift to one side or the other.

About 3 weeks ago, my right front brake caliper seized up while I was driving around town; I limped home and ordered parts. Once parts arrived, I bleed the brake lines, installed new Front Right parts and rebleed the 4 brakes. Took my car for a test spin around the block & noticed smoke from the rear -- turns out the Back Right caliper was now seized. I decided to replace the 3 remaining calipers + brake pads, so I ordered new parts. The Front Left and Rear Left calipers installed fine, but the Rear Right caliper kept hitting the rotor on its inside, like something was out of alignment.

----- Observation -----
The caliper wouldn't mount because it hits the rotor.
Initial observations seemed that the caliper was not machined correctly post casting. The new caliper had far less wiggle room between the two sides like they had not done all the post-cast machining, or there was some junk on the fixture so the caliper wasn’t sitting flat when it was machined.

As a curious engineer, I removed the Left Rear caliper & compared the two new rear calipers -- they looked like similar reflections (same model). I even tried the new Right caliper on the Left mounting, and surprisingly it fit fine. I was frustrated and confused but since the original Right Rear caliper mounted fine (but has a seized piston), I was certain that the new Right Rear caliper was defective. So I sent it back and ordered a new one.

Backstory: When getting my parking brake functioning for inspection, I ordered 3 different pairs in succession because it turns out they were not being manufactured correctly (like mis-reading drawings or something). I found a random blog that documented this problem and a hack fix. This prior difficulty made me quick to judge the non-fitting RR caliper.

----- Round 2 -----
The new Right Rear caliper came in today, and again, I could not install it because the inside of the caliper was hitting the rotor in exactly the same manner as the caliper I sent back. This threw up flags and made me question my prior judgement. [scratch head for a long while]

-I again removed the Left Rear caliper to compare the two, and again, they looked like solid reflections.
-And again, the Right Rear caliper fit on the left mount with correct rotor clearance on both inside & outside.
-I attempted to mount the Left Rear caliper on the right side, but could NOT because it hit the rotor in the similar manner as the two replacement Right Rear calipers.
-The Rotor is mounted as it should be, and without any junk behind it as an offset.


----- Deduction -----
My current deduction is that the Right Rear caliper mounting tabs (somehow) got slightly bent.

-Bent mounting tabs would explain why the two new Right Rear calipers and the new Left Rear caliper can NOT be correctly mounted on the right side, but COULD be mounted on the left side (with appropriate rotor clearance for all 3 instances).
-Also, since the original caliper has more clearance than the new calipers, a slight angular misalignment wouldn’t cause interference.
-Lastly, when the Right Rear caliper is centered about the rotor (but at a further radius from the axle), it appears that the caliper mounting tab thru holes and the car’s mounting tab holes are at a very slight angle (this is shown in a picture below).


----- Action -----
I feel like I already jumped the gun misdiagnosed this once, so I would appreciate any input, insight or feedback, and most desired, suggestions on how to proceed with how best to address this issue.

-There are probably tests that I can do to verify this (assumed) problem. If anyone can think of anything, please toss out ideas.

-I initially thought that the caliper hadn’t been post-cast machined correctly, so I was going to split the caliper in two, machine the extra material away to give it the clearance of the original caliper, reassemble, and hope the install works without compromising structural integrity. But since the two Right Rear calipers both hit the right rotor in a similar manner, and they fit correctly on the left side, my confidence is LOW that the caliper is the problem.

-While writing this up, I also thought that maybe the rotor became warped from heat while doing my test drive.
Visual inspection: They look like they are in good shape, have little wear, and look pretty damn square (I can check in the AM). I can try swapping the Rear L&R rotors and see if that makes a difference, but honestly I don’t think it will.

Q: What am I forgetting / overlooking?

[awcarvey]

----- Old Caliper Right -----

~19mm mount surface to inner edge
~35mm mount surface to outer edge

----- New Caliper Right -----

~19mm mount surface to inner edge
~32mm mount surface to outer edge


----- New Right Caliper on Left Side -----

Note that there is visible clearance between right caliper and left rotor.

----- New Right Caliper on Right Side -----

The new Right Caliper centered about the right rotor (as it should be (and was on the left side)).

When the Right Caliper is centered about the rotor, the mount tabs are not co-planar (slight angle).

I could not fully mount the Right Caliper because it would hit the rotor such that the mount tabs are at a distinct angle.

[awcarvey]

Took some more measurements.
The distance from the mounting tabs to the inside of the Rotor is ~24mm and the Rotor is 10mm thick.
The distance from the mounting surface of the caliper to the inside slot edge is ~19mm (so under 24mm), and ~32mm on the outside edge (unfortunately under the 34mm required for rotor clearance). Those 2mm of overlap collision is what is making them not fit. <grrrr>

The calipers are NOT fabricated correctly

Going to return them and look into rebuild kits and calling local shops.

[awcarvey]

Further investigation suggests that the new calipers ARE fabricated correctly.

I determined that my axle bearing has a 2mm gap between it and the axel's shoulder (which it should be flush against).
The 2mm hopefully will allow the caliper to be correctly mounted.
Time to find a press!

[abscate]

Glad you hunted this down. I’m dealing with a bent caliperbracket in a Vw at the moment. I’ll have to heat with a rosebud and straighten , or o replace the entire rear axle and suspension