Plus 2 rear brake disk wear

[JonB]

Hi

An incidental question relating to my back end work. I noticed that the rear brake disks do not appear to have much wear on them. In fact, there is still some rust on the surfaces from before I owned it. I've done around 300 miles, and I would expect the pads, which have plenty of wear left in them, to have cleaned the disk surfaces by now.

I've also noticed a tendency for the car to lock up the front wheels, especially in the wet.

This leaves me wondering: do I need to alter the braking circuit so as to apply more bias to the rear brakes? As far as I'm aware, there is no way to do this, it being a single circuit system. Yet I was not at all happy about the front lockup in the wet, very dangerous. Seems to me that if there was more bias towards the rear of the car, it might have slowed down better (because it would be taking better advantage of the rear tyre grip).

I'm going to have a look at the archives - in the meantime, any ideas / advice?

Cheers
JonB

[rgh0]

Braking is a plus 2 is pretty balanced, if anything the rears will lock up with modern tyres
Your rear pistons may be sticking in the bores - time for an overhaul I think unfortunately

cheers
Rohan

[alan.barker]

They all do that Sir
The 3 different +2 Elans i have had did that, 1968 +2, 1970 +2s and 1973 +2S130/4.
In the wet with a +2 never ever stamp on the brakes and above all not on a corner.
The brake pedal is very light.
Alan

[alan.barker]

Maybe once you have cleaned the rear caliper pistons or fitted stainless pistons, you could fit the original dust shields to the wishbones.
Alan

[MarkDa]

Yes Jon, the old chromed pistons are prone to corrosion. Easily solved with some nice new stainless ones.
I put some in my car 35 years ago and despite the car bein laid up for 30 years they were still free moving.
It's only a '10 minute' job to replace them.
On the other hand that rebuild I warned you about might just happen!
With the rear brakes working the load on the fronts should reduce and so the tendency for them to lock.
Mark

[john.p.clegg]

Jon

Been there,was that man , you could always fit ABS....

John

[paddy]

In fact, there is still some rust on the surfaces from before I owned it. I've done around 300 miles, and I would expect the pads, which have plenty of wear left in them, to have cleaned the disk surfaces by now.

It depends on the nature of the corrosion on the disc. It's not uncommon, especially on cars that don't get out much, for there to be a band of corrosion on either the inner or outer edge of the disc braking surfaces; This is so abrasive that it quickly wears a step in the pad, and from that point on it is never cleaned by the pad. The width of the corroded band progressively gets wider and takes out a large fraction of the braking surface, resulting in a loss of efficiency. The only remedy is to have the discs replaced or re-ground to a clean flat surface - no amount of driving with it, even with new pads, will clean it up. There's an example here: https://postimg.org/image/id2legss1/

Paddy

[JonB]

No, it's not that thick layer round the edges, it's in the middle of the rotors and a very thin layer, like you see on a car that's been wet and stood for a week or two. Surface rust only. Point is, I expected that to be worn away first time I use the brake.

[RichC]

yep sounds like seized calipers to me ....

[MarkDa]

Both sides of disc?
You may get lucky by pushing pistons back manually a couple of times to free off as a quick fix.

[alan.barker]

I suggest BEFORE you push the pistons back in remove the brake pads. Put something thinner than the pads inbetween piston and disk. Apply brakes to get the pistons to come out a little and clean the part of the piston you can see. Then you can push them back in with out tearing the seals
Alan

[jono]

I would also suggest you open the bleed nipples when you push the pistons back

I failed to do this and ended up wrecking the master cylinder seal.

[MarkDa]

Good advice x2
Doing that will also give you a good idea of how bad the pistons are.
Go steady easing the dust boots so if pistons are ok you'll have something left to protect.
To ensure no back flow I usually clamp the flexible connection.
In fact I do this every time I put new pads in - it's a kind of partial fluid renewal.

[alan.barker]

I would also suggest you open the bleed nipples when you push the pistons back

I failed to do this and ended up wrecking the master cylinder seal.

Strange I've never done that and no problems with the master cylinder.
How can pushing the pistons back damage the master cylinder seal.
Alan

[jono]

It's an interesting question Alan, and one I don't fully understand myself, other than it happened to me after I changed my fluid and with a quite recent, new, master cylinder.

I could not understand why I could not get a pedal after doing this (and changing the rear discs).

A internet trawl showed this up as a common issue but, like you, I can't understand why but I tend to learn from experience rather than qeustion too deeply. I fitted new MC seal and all was immediately well.

It's said that reversing the pressure causes the seal to 'flip' or otherwise be damaged but I can't really understand how that might happen so you raise a fair point.

But for sake of cracking the bleed nipple open when pushing back I always do it now as once bitten twice shy