14 replies to this topic

[mattcave]

I've just replaced my 8.4" disks/calipers with a (brand new) 7.5" setup. I'm struggling to bleed them. The lines all seem OK, no leaks. There's no air in the rear lines, I'm confident of that. It's a 1993 SPi car with servo. Master cylinder was fine before the conversion.

 

When I open the bleed nipples I'm getting clear fluid through, no air bubbles. However, when my assistant depresses the brake pedal, I can see the pistons on the front callipers move inwards few millimetres, and when the pedal is released the pistons retract again - so I never get a solid pedal as the pistons/pads never make contact with the disk face. I can't remember coming across this problem before. What's going on here, and how do I fix it?

 

Thanks!

 

[mini13]

try wedging the pedal hard down with a peice of wood under the steering wheel over night. it may be that the seals just need " helping"

[nicklouse]

yes pistons do retract when the pressure is removed. 

 

how do you mean

 so I never get a solid pedal as the pistons/pads never make contact with the disk face.

 

they have to when you press the pedal.

[Northernpower]

I've just replaced my 8.4" disks/calipers with a (brand new) 7.5" setup. I'm struggling to bleed them. The lines all seem OK, no leaks. There's no air in the rear lines, I'm confident of that. It's a 1993 SPi car with servo. Master cylinder was fine before the conversion.

 

When I open the bleed nipples I'm getting clear fluid through, no air bubbles. However, when my assistant depresses the brake pedal, I can see the pistons on the front callipers move inwards few millimetres, and when the pedal is released the pistons retract again - so I never get a solid pedal as the pistons/pads never make contact with the disk face. I can't remember coming across this problem before. What's going on here, and how do I fix it?

 

Thanks!

 

So what happens when you pump the pedal, does it then go solid?

[mattcave]

Push pedal to the floor, calliper moves in, but not far enough for the pads to contact the disk.

Release pedal, calliper retracts, back to where it started.

 

Repeat 10 times, calliper still ends up where it started (ie almost fully retracted).

 

Make more sense now?

Edited by mattcave, 01 February 2018 - 04:46 PM.

[Northernpower]

Push pedal to the floor, calliper moves in, but not far enough for the pads to contact the disk.

Release pedal, calliper retracts, back to where it started.

 

Repeat 10 times, calliper still ends up where it started (ie almost fully retracted).

 

Make more sense now?

No, what I meant was, if you pump it quickly, does the pedal then go solid?

[nicklouse]

sounds like the valve is cutting in thinking you have a leak.

[mattcave]

No, what I meant was, if you pump it quickly, does the pedal then go solid?

 

Ah, gotcha. Just tried that...

 

Pump it 4 or 5 times, and the pedal does indeed go hard. So that's good! And if I hold the pedal down, it stays hard, doesn't slowly go down to the floor, so that confirms there are no leaks.

 

If I take my foot off the brake pedal for 5 seconds or so, the pedal then needs 3 or 4 pumps for it to go hard again. I guess because the pistons are slowly retracting again?

[mattcave]

sounds like the valve is cutting in thinking you have a leak.

 

Yeah, and I can see why it "looks" like a leak. There's resistance in the rear circuit, but not in the front circuit. Just not sure how I would go about resolving that. 

[panky]

Just thought I'd suggest the obvious one - are the bleed nipples are at the top of the calipers?

[mattcave]

Just thought I'd suggest the obvious one - are the bleed nipples are at the top of the calipers?

 

They are, yeah, thanks for checking though.

 

If there was still air in the system, the pistons wouldn't be moving.

[Spider]

I think you'll find the seals are dry. They will retract a little in normal use, but not to the extent you are seeing and this came up elsewhere recently.

 

The fix is easy.

 

One at a time, remove a pad, pump the piston out 4 - 5 mm and push it back, replace the pad. Repeat this with all Pistons.

 

This will lubricate the seals with Brake Fluid and allow them to settle where they need to.

[mattcave]

I think you'll find the seals are dry. They will retract a little in normal use, but not to the extent you are seeing and this came up elsewhere recently.

 

The fix is easy.

 

One at a time, remove a pad, pump the piston out 4 - 5 mm and push it back, replace the pad. Repeat this with all Pistons.

 

This will lubricate the seals with Brake Fluid and allow them to settle where they need to.

 

Nice! It will have to wait for the weekend, but I'll give it a go and report back. Does sound plausible/sensible!

[nicklouse]

 

sounds like the valve is cutting in thinking you have a leak.

 

Yeah, and I can see why it "looks" like a leak. There's resistance in the rear circuit, but not in the front circuit. Just not sure how I would go about resolving that. 

 

gravity bleed or just press ridiculously gently and slowly.

[mattcave]

To close the loop on this one, the explanations / solutions proposed by Moke Spider and mini13 seemed to do the trick. Wedging the pedal down overnight improved things significantly (and proved what the cause was), and Moke Spider's solution nailed it properly. 

 

Thanks both!