312mm brakes doing my head in

[retro]

I fitted Octavia VRS 312mm brake calipers and carriers which bigg red shot blasted and cermaic bake on painted then I rebuilt with a seal and piston kit from them. 

 

It has been bled with VCDS but they feel rubbish, not much happens at the top of the pedal and then when you really stand on them they don't inspire confidence. Even the knackered 288mm I took off were way better than these. 

 

I almost knocked a cyclist off as I was doing 40mph and they randomly decided to veer across the road in front of me with a car coming the other way! The brakes took forever to stop me. 

 

I also fitted new Octavia front flexi hoses, rear discs and pads are in good order and the fronts are Brembo discs and pads not cheap junk, I can't see any leaks anywhere.

 

Has anyone got any ideas on this one? I'm more used to stuff from the 80's not all the computerised rubbish (what ejit designs a car you need a computer to bleed?!?!). So I can't get my head around what could be up other than it needing another bleed as the dead spot suggests to me. 

[sepulchrave]

If the brakes pump up then there is air in the system, if the pedal doesn't sink then there isn't.

Are you bleeding the brakes using a pressure blender? In my experience you can't bleed them without one.

[StevesTruck]

You don't need a computer to bleed the brakes unless you've got a reason to think there's air in the ABS pump, just do it like you've always done, then bleed the clutch at the end (it's not in the circuit, but it shares the reservoir). It's worth pre-filling the calipers with fluid if you can as a shortcut, but as they're already on, bit late now. 

 

I've personally never got on with pressure bleeders, if you're on your own, I've always found a vacuum bleeder is a far better way to go about it, especially if you're filling an empty caliper, as it doesn't push the pistons out. I use a Sealey one like this (compressor required).

 

https://www.ebay.co.uk/itm/Sealey-VS020-Air-Brake-Clutch-Fluid-Bleeding-Tool-Vacuum-1ltr-Car-Van-Bike/130649782649?epid=12031170110&hash=item1e6b557979:g:OcUAAOSwFvZbnGMA

 

All the obvious stuff applies, check the whole system for leaks, make sure the rear flexis aren't ballooning up under pressure, make sure your pads aren't contaminated.

 

If you think there's a chance the fronts aren't filling right when you're bleeding them, it's a dirty trick, but I have been known to slide a shim in between the disc and the pad, so the pedal effort all goes into pushing air out, rather than actuating the brakes.

Edited 6 January, 2020 by StevesTruck

[retro]

I bled with VCDS as I changed a rear caliper a few days after fitting the 312mm upgrade and I couldn't get any pedal so got someone from eBay to come over with his laptop and we bled the brakes, it was a massive improvement but still not right.

 

I was trying a gunsons ezi bleed before that.

 

 My mate has one of those vacuum bleeders, I'll give that a go and the shims if another go with the ezi bleed doesn't work. 

[StevesTruck]

Yeah, those Gunson kits are alright for changing fluid. Plenty of people will disagree with me, but I've never got on with them for filling empty calipers.

 

You won't need to shim it out if you're using the vac bleeder as it will naturally pull the caliper pots rather than push them. You're not trying to bleed with the handbrake set are you? Did you wind both rears back in, or just the one you changed? Again, dirty tricks, but I've been known to leave the windback tools on the rears until the caliper are bled, if I'm bleeding them with the pedal, then fit them properly. 

 

I'd personally wind the rears back. Vac bleed it all round, then give the pedal some use to wind the rears back out.

Edited 6 January, 2020 by StevesTruck

[J.R.]

You do realise that the calipers are handed and that if you fit them to the wrong side you will never be able to bleed them dont you?

 

Apologies for reaching you to suck eggs if you did know but pretty much every case I have known of poor brakes and problems bleeding after a caliper change has been down to that.

 

Somewhere I have a picture that I took of the brake caliper of a Piper light aircraft that took me to Canaima camp in Venezuela, I knew that it had no brakes and the pilot suddenly could not understand English any more when I asked him, a shame because I could have told him how to get them back.

[retro]

On 06/01/2020 at 22:05, J.R. said:

You do realise that the calipers are handed and that if you fit them to the wrong side you will never be able to bleed them dont you?

 

Apologies for reaching you to suck eggs if you did know but pretty much every case I have known of poor brakes and problems bleeding after a caliper change has been down to that.

 

Somewhere I have a picture that I took of the brake caliper of a Piper light aircraft that took me to Canaima camp in Venezuela, I knew that it had no brakes and the pilot suddenly could not understand English any more when I asked him, a shame because I could have told him how to get them back.

Yep, if they went on the wrong side the bleed nipple would be at the bottom.

[retro]

On 06/01/2020 at 20:24, StevesTruck said:

Yeah, those Gunson kits are alright for changing fluid. Plenty of people will disagree with me, but I've never got on with them for filling empty calipers.

 

You won't need to shim it out if you're using the vac bleeder as it will naturally pull the caliper pots rather than push them. You're not trying to bleed with the handbrake set are you? Did you wind both rears back in, or just the one you changed? Again, dirty tricks, but I've been known to leave the windback tools on the rears until the caliper are bled, if I'm bleeding them with the pedal, then fit them properly. 

 

I'd personally wind the rears back. Vac bleed it all round, then give the pedal some use to wind the rears back out.

The new rear caliper was already wound back so I just fitted it and didn't touch the other side. Took absolutely ages to get fluid coming through it with the ezi bleeder.  I'm gonna borrow my mates vacuum bleexer and see if that improves them. 

 

Just thought I would add that the brakes have done about 2k miles maybe more so the discs and pads should have bedded in by now.

Edited 8 January, 2020 by retro

[Guest BigJase88]

try bleeding again and if no better you may have a vacuum leak on the pipework to the master cylinder, that's what was on my mates audi tt, pipe had perished and was leaking air

[Sirlimpsalot]

Is this a common swap? (Sorry new here)! If not could it be a master cylinder ratio problem? A master cylinder that has too small a fluid volume won't push enough fluid to move the pistons out as much as needed.

[sepulchrave]

15 minutes ago, Sirlimpsalot said:

Is this a common swap? (Sorry new here)! If not could it be a master cylinder ratio problem? A master cylinder that has too small a fluid volume won't push enough fluid to move the pistons out as much as needed.

 

It's such a common mod amongst VAG group cars that 312mm calipers and carriers are getting hard to find secondhand these days.

 

Welcome to the forums new guy, nice thought but you're barking up the wrong tree since the calipers are basically the same, it's only the discs that are bigger.

[KenONeill]

@Sirlimpsalot - The calipers are literally the same ones as used on pretty much all VAG cars with 288mm or 312mm discs. In fact, if you had had a car with 288mm discs, you'd only have needed new discs, pads and caliper carriers. Actually, that gives me an idea! 💡 When were the pads last changed, because used pads won't bed in to the new discs ever?