Search all blog posts here.

Search This Blog

Sunday, September 21, 2014

Shovelhead Rear End Re-do. Part 3: GMA A Caliper Rebuild/Rework

As always, this post has some proven things in it and some things that may only have "seemed like a good idea." I'm sure someone will email me and tell me why all of this won't work.

I lost all the fluid out of my rear brakes twice within the last month or so. I did a new set of pads, o-rings, and bleed nipple the first time. The second time I realized how warped and messed up the rotor was. It was clear the piston had cocked, too.

I opened the caliper halves and noticed these rub spots from the pad's backings.

All sides were a couple of thousands deep and a bit rough,

So I decided it couldn't hurt anything to file those smooth and flat again.

With some patience and work the line starts to disappear.

I got about all I could on both ends of the outboard side. A little bit was left, but that would be a bunch more filing.

Now to the inboard side. Same rubs, but the pins are in the way of the file.

These are basic roll pins, so removal in hard.

A little work with a roll pin punch and hammer.

Results of the file work.

I removed this elbow and cleaned the crud out of the threads.

I put the pin back in, but that was dumb. I should clean those up, too. Replacement pins are available, but I can't seem to find a source.

Back out.

Clean them up.

Still some rub spots, next time I will find new pins for sure.

That's interesting. An 86 vibrapeened right there. I have never noticed that before.

Replace the pins.

Tape the elbow.

Clean everything to get the filings off.

Pop the pistons out.

Pull the o-rings.

A and B kit is the same thing with a little o-ring you won't need.

Get the new o-rings in and I figure I would go ahead and lube them with DOT 5 and install the pistons.

Find the o-ring that fits this counterbore and use it. The other one is for the other style caliber.


Just the roll pins nothing else.

Looking good.


Second pad.

Outboard body.

Again I talked with Ed about this and he's correct, the piston should be taller. When the pads and rotor get thin, the piston push out too far and get cocked, then jam or dump the fluid. There is a lot of slop between pads and rotor with the piston pushed all the way in. Note: the elbow points out a bit to help route the brake line off the tire a bit.

My plan is to use shims between the pad backing and pistons. That should keep the piston deeper in the bore. My initial thought was to knock the friction material off a set of pads and use the backing for a shim. Before I put the work into cleaning off the friction, I did a quick mock-up. The backing is about 0.125 inch and that is too thick.

So Plan B is shims from .063 Aluminum stock I have on had. The old pads will still be the pattern though.


Scribe it out.

Transfer punch.

Drill and oblong it a bit.

About right.

Deburr the holes.

Cut out the shape.

About right.

Sand a bit to round off the corners and edges.

I built two of course and did a fit check. One was tight, but a little work...

And it was good to go.

New stack up with shims behind the pads.

Fits tight on the rotor, but still have some room to move. If I had the correct new tire, I would have this thing together. I still need to inspect the master cylinder to make sure it's releasing pressure properly, too.

No comments:

Post a Comment