Front brakes mk1

[Jonny-Jimbo]

Hi all,

As part of my prep for HCR I tried to replace the front brake pads, but I had some issues.

I tapped the pins back out, and lifted off the retaining springs.
Then I have to use a punch to tap the old pads out of the caliper, but once they had shifted a touch they came out okay.
Then I took the cap off the master cylinder, and used a g-lamp with a plate to push the pistons back in. The outer one went in fine, the inner one kept bouncing back a little, but only a fraction of a mil.
Seeing how rusty the caliper was I took the bolts out, caliper off and gave it a good wire brushing, and cleaned the disk up at the same time.
Gave it a spin and couldn't see any obvious deviation in the disc.
Caliper back on without too much issue, just aligning the caliper, damper, knuckle and dust shield was a pain in the backside.
Got the bolts back in, torqued them to half tightness just in case it had to come off again, but no wobble.

Then came the issue; I could only just get the new pads in with the shims. I have two sets, a NOS set of standard material, and a set in DSII material. I could not get either of them to fit! They would push in fine, but then I had to give them a tap to seat them. The inner pad just wouldn't go in at all, without using more force than I'd like. Once they were in, the disc was jammed absolutely solid, and even with a big lever bar across the studs I couldn't turn it.

I tried a combination of the pads I had, realising the backplates aren't symmetrical, but nothing would fit properly.

I wonder if anyone can give me a pointer of what the problem may be? I've done loads of brake swaps in the past, but never had this issue. I couldn't see what they were fouling on, other than the disc itself.

Oh, this is on a car will running the thinner discs.

In the end I had to refit the old pads. Not ideal, but they should see us through HCR, but will definitely need doing before RBRR.

[mikeyb]

Assuming similar to Stag/Mk2 you should not have 4 identical pads - instead 2 pairs.  I have encountered new boxed pads of several brands including Mintex where the pads are all identical and therefore two will not seat in the caliper properly.  This can be easily sorted with a file -  putting a pair back to back will show you what needs to be removed.

If you cannot get a pad in because of the clearance between disc and piston, then assuming the pad is not of incorrect application, then simply your pistons are not retracting fully. Have you tried "g-clamping" them in with the bleed nipple open?

[JohnD]

JJ,
" The outer one [brake piston]went in fine, the inner one kept bouncing back a little, but only a fraction of a mil."

Pistons rebound slightly due to the seal around them in the caliper. That's what pulls them back from the disc when you let go of the pedal.
If one doesn't rebound then perhaps it has a worn seal.
If the other rebounds excessively, then I think you should inspect and renew the flexible brake hose(s).    Internal degredation may have formed a one way valve, impossible to detect externally.

John

[Jonny-Jimbo]

As I said, I have NOS Lockheed pads to replace the old ones with, with new shims too. I could see the backplates of the pads were not symmetrical, but I tried them in a few different combinations. One type of pad in the NOS set were marked '.450' and another '.473' which I guess are the friction material thicknesses? Either way, I couldn't get them to seat correctly.

I had the pistons pushed in flush with the caliper body, so no reason for there to be an issue there at all. As I said, I had the caliper off and cleaned it and the disc.

My intention was to just do the pads for HCR, then do a full brake rebuild on the front for RBRR, new braided lines, possibly even upgrade to bigger calipers and discs etc.

[thebrookster]

Johnny,

May I enquire as too which NOS pads you are using?

I recently bought a set from Chris Witor, NOS etc. However it turns out that whilst they are in fact NOS pads, they are not the original pads spec'd for a Mk1 (which near enough no longer exist in any quantity), but a style of pad that is a very close fit.

The pad shape is fine, however the thickness of the pads is greater than the "real" originals, so you need to grind the back of the pad (backing metal) down a few mm in order to get them to fit.

Is this the issue you are having? I initially found out through one of the forums, however Mr Witor happily confirmed the same.

Cheers,

Phil

[dazzer]

Could be a build up of rust on the inner side of the disc inner edge. Put a nice smear of copper slip on the back of the pads and a slight smear on the top and bottom edges tap them in if they are a bit tight and also forget your G clamp to push the pistons back. You can lever them back with a big flat screw driver or lever and see that the piston is actually fully retracted. Do one piston get the pad in then do the other piston. Copper slip the pins as well. Get some P40 on the discs as well to break the glaze whilst its apart. The new pads will bed in better. You wont break or distort anything levering the pistons in... well i guess theres always the first time! but ive been putting them in like this since my BL days and not bent or broken anything yet in 40 years.

[thescrapman]

I wonder if the bottom of the pad needs to be shaped to match the curvature of the hub or disc at the inner edge.

Perhaps try bolting your skinny mk1 discs to some mk2 hubs.

[DJB_Harvey]

Sounds to me that your stub axle spider is worn out ... That's normally the problem when you can't get a new pad in on the rear face .

[Jonny-Jimbo]

I bought the NOS pads from Chris Witor - the DSII material pads, which are also Lockheed were just ones I've had stashed away for years - can't remember exactly where from though. I would say I would rather rub down the friction material on the pad than take metal off the backing plate, but with it being a NOS type pad, and therefore a bit asbestos-y that may be a bad move! I did a fair bit of work on backing plate strength and pad chamfer angles etc at the Nurburgring, and the thinner back plates were generally a no-no in terms of pedal feel.

I'll have to leave it until after HCR now and just hope that we're kind to the brakes ("How're the brakes?" "I don't know, didn't touch them." etc). Either way, I think it needs some further investigation, so I will have to see what's what. Maybe drop the hub off the spindle too so I have better visibility of what's going on. Maybe change the discs too...

[Jonny-Jimbo]

What wears out on the Spider Dave??

[DJB_Harvey]

The spider wears where the inner bearing in the hub sits ... It wears a groove so slowly the disc moves further to the spider ... Bad ones mean you can't get a new pad into the calliper on that side .

[Jonny-Jimbo]

So would that mean changing the sub axle would be acceptable, or do you mean the  face of the upright is worn away? Would this not cause play in the bearing on the stub axle, as I couldn't detect any play there.

[DJB_Harvey]

The area marked with the arrow is where the wear takes place .. There won't be any more play in the bearing because you can adjust that out .
Take a look at where the disc is running in the calliper it should be central , but if the spider is worn ( not the stub axle ) the disc will be running to one side . EG towards the spider .

[Jonny-Jimbo]

Ah, I see. I didn't have any play in the bearing that I could detect, but will check again more carefully now I know that., and will also check that the caliper is centralised over the disc. In my head it was, but will check.