420 rear calliper removal – any advice please?

[BookerME]

What a fantastic piece of engineering the IRS unit on my 420 is but what a total nightmare to work on!

I need to remove both callipers, so the whole IRS unit has had to come out. I thought after that, the callipers would simply unbolt – oh no.
It would seem to remove the callipers, I’ll have to remove the discs and callipers together as the 2 calliper mounting bolts are not accessible by any means.
To remove the discs it would seem I’ve got to remove the drive shafts then somehow turn the discs so the holes in them match up with their two mounting bolts.
Before I can do any of this I need to remove the old brake pads from the callipers. However, the outer piston is seized, a crowbar in the gap won’t move it back. There’s no access from the back to knock the pad out and the pad is jammed tight against the disc.

So, two things I’m looking for advice on.
1. How can I get the jammed pad out? My only idea so far is to chisel off the brake pad material. Anyone got any better ideas?
2. Am I missing anything with the calliper removal that would make it easier to get off without removing the drive shaft and disc?

[piman]

Hello BookerME,

it will help if you can replace the old pad on the opposite side and also pack it so it is hard against the disc with thin steel strips, so as to minimise deflection of the disc itself. I would then try an old broad wood chisel or similar to drive beteen the pad and the disc. Also soaking the caliper area with Plus Gas where the pad sits may help somewhat.
A last resort is to drill holes in the pad material and chisel it out

Alec

[PPV]

I found when I did mine that constant tapping the pad backing backwards and forwards (and side to side) after spraying the piston with plus gas eventually got it to move sufficiently to pull it out. It took quite a bit of time though so be patient.

[BookerME]

Thanks chaps, all good suggestions. I did think about drilling but I'd need to be extra careful not to catch and damage the disc.
I'll treat myself to a can of Plus Gas, I haven't used it for years having relied on good old WD40 but it may have just a bit more oommf about it to ease the way.

Cheers, Mac.

[PPV]

Hi Mac, WD40 does little in circumstances like this. Best of luck.

[BookerME]

All sorted.

A combination of heat, a gentle hammer, PlusGas, chisel and pliers eventually got the jammed pad out.

Caliper removal (caliper with one ‘L’ – having had my spelling corrected!) is also sorted.

The workshop manual is misleading. If you ever have to do this job, the steps are:-

Remove the handbrake caliper.
Unbolt the UJ / drive shaft at the disc and move the hub carrier out of the way. Be ready to catch the shims between the UJ and the disc.
Remove the 3 (yes, 3) bolts holding the mounting plate to the diff by putting a socket through the hole in the disc itself. Obviously you have to rotate the disc to do this.
Once you’ve moved the disc and mounting plate, you’ll have enough room to get a ring spanner on the two bolts holding the caliper to the mounting plate and also enough room to withdraw these bolts without them fouling on the diff.
You can then juggle the caliper out of the gap at the front of the IRS unit.

Don’t forget to disconnect the brake fluid pipe from the caliper.
You don’t need to remove the front spring/damper as the manual says.
It would seem the disc itself won’t actually come out without removing either the wishbone or the disc mounting studs – but you don’t need it out, you just need to move it enough to get access to the caliper mounting bolts.
Watch out for a couple of large washers between the inside of the caliper bolt holes and the mounting plate, they may fall on the floor leaving you to wonder where they’ve come from.
All the bolts may have wire locking on them which’ll need cutting off and removal before putting a socket on them.
Cheers, Mac.

Here you can see why the caliper won't unbolt with the mounting plate bolted in place. There's not enough room to remove the bolt, even if you are lucky enough to get a spanner on it in the first place. This one was comparatively loose compared to the other three.

[davidr]

Well done! It all looks easy enough in the manual but it's overcoming these things where something is stuck solid that give the real challenge.
If you're thinking of going a bit deeper now your IRS is on the workshop floor you might find the thread where I rebuilt the IRS on my XJS interesting. There will obviously be differences but the principles are basically the same. It's in the Suspension section of the XJS Forum. I'm humbled to see that it has now been viewed over 20,000 times! I hope you find it useful.
Regards
David

[piman]

Hello BookerME,

some jobs are like that, even for professionals, it requires, as you found out, perseverance and often trying a combination or different approaches to get the job done. First principles do apply, such as hammering or levering against something, it needs to be solid for maximum effect or indeed in some cases any effect at all.

Alec

[ChrisDixon]

A fun old job, but it is possible to remove in situ as I recall, especially if someone has kindly cut a hole in the boot floor to facilitate maintenance, but as others have commented you'll find other things to do if you drop the IRS, which is actually easier having done it both ways on different cars. Interestingly when I did it on my 3.8S leaving the cage in place, which meant removing the lower arms, I found a number of spacers missing, which I wouldn't have spotted if I'd just dropped the cage and only unbolted the driveshafts. The reason I fitted the discs and calipers afterwards was due to the assembled weight being too much for me to refit the diff with them on single handed, and I recall having to remove the caliper bolts to insert shims to get the caliper centered so the pads fitted, then try and fit lock wire!

[BookerME]

Thanks Chris, I eventually got the calipers off. As you say, a fun old job. I struggled even with the cage out as the various bolts were so tight but I got there. In comparison, the front calipers were a breeze, I had both of those off in about 20 minutes!
That IRS unit is a fantastic piece of engineering design for its time but what a nightmare to do any work on.

Anyway, it turned out that taking them off was the right thing to do as one of the pistons was seized solid.

Reinstallation is likely to be just as challenging of course but I’ve only just recently found the 4-part article in the mag by Bob Bate which at least gives me a decent guide and extensive check list to use – even if it doesn’t reduce the weight of the thing!