Rear brake backing plates for MGA. Difference between left and right is orientation (clocking) of the four mounting holes at the center hub.

Notice around the rectangular slot there is a thin steel shim, spot welded to the main plate. This provides a bearing surface, elevates it slightly, and assures correct overall thickness to mate closely with the sliding slave cylinder.

There is a tech article under Faulty Parts showing the small sliding gap in the slave cylinder illustrating why this thin plate dimension is important.

Here we have pictures of corrosion on the backing plates where rust under the thin shim plate is lifting or expanding the shim. Corrosion should not get this bad when parts are in assembly, but when the slave cylinder is removed such corrosion can affect the shim plate much more.

When corrosion adds rust thickness and distorts the shim plate, the slave cylinder may no longer fit in the slot. Or if it does get into assembly it may bind up and not slide as it should. Being welded in place, these shims were never offered as replacement parts, so if you grind them off you have to make your own to finish the repair. And unfortunately these backing plates are not currently available as new replacement parts.

There have been suggestions to hammer the parts down flat, maybe using penetrating oil in attempt to displace the rust under the shim, sand, file grind to get it back to flat and smooth. Chemically etching the parts might remove most of the rust (maybe), similar to having rusty body sheet metal dipped and etched down to bare metal, then maybe use epoxy steel filler to smooth out the pit marks.

You could file new wheel cylinder to enlarge the space to accommodate the fit, but then you can never again buy a replacement slave cylinder without having to modify it again. When corrosion is this bad, it may be more expedient to look for other good used parts in better condition.

David Adams measured the plate to be 0.100" - 0.103" thick (depending on paint and rust), and the shim being 0.0438 thick (19 gauge), total 0.144"-0.147" thick. The slider space on the slave cylinder is 0.157", giving running clearance of 0.010"-0.013". Looks like a new shim plate could be made from 19-gauge metal, to be spot welded or soldered in place. I prefer soldering for permanent installation and corrosion resistance. Another suggestion is to make the shim a little longer and maybe attach it with pop rivets.