[TurboWeasel]

Originally Posted by Ennoch

In theory, yes. However, I remain to be convinced that you can adequately loosen and tighten to get a good bleed, especially if pumping the pedal rather than using pressure bleeding (and I'm not a fan of the latter either). Even with the stock soft pedal on the BMW I've re-bled my brakes immediately subsequent to BMW doing it and managed to improve the firmness both times. The fluid that ages most is the stuff in the master cylinder, because it's not sealed, and in the calipers, because that shit gets cooked (or at least it does when it's in a car I own!). I've never used ISTA but I'd not be concerned with not cycling the ABS pump, unless somehow you've managed to suck air into it. For standard fluid refresh I'd not bother, and not be remotely concerned.

Pretty much every 4 piston brake will have two bleed nipples on a car. MTB/motorbike ones don't because they tend to have the entry from the line at the top of one side (where nipple #1 is on our M calipers) and then exit through the single nipple (where nipple #2 is on ours). Because of clearance issues on auto stuff you have the line going into the back face of the caliper, hence the two nipples. My AP's are identical in this regard.

Cheers for the detailed response and insight as always.

You're right, I just looked up the AP Racing CP 5200 calipers I used to have and they do indeed have 2 bleeders. It was 14 years ago, so foggy memory and all that.

[ian332isport]

Originally Posted by TurboWeasel

Not sure why those calipers have 2 bleed nipples? I'm sure my old AP Racing calipers had 1 nipple and a cross tube linking the two halves.

As Ennoch has pointed out, you hardly ever get 4 (or more) piston calipers with a single bleed nipple. Some calipers have internal drillings to link the two halves of the caliper and some have an external link pipe like your old AP Racing calipers, but the principal is the same.

Even if you made a caliper with both upper and lower crossover pipes (and I think they do exist), you still wouldn't be able to bleed the outer half of the caliper properly. As with most things, the fluid is going to take the path of least resistance, so if you had a single bleed nipple on the inner half, the fluid is going to flow from the fluid inlet directly up to the nipple. You will get virtually no fluid flow through the lower crossover, back around through the upper crossover and finally out of the nipple.

If the single bleed nipple was on the outer half of the caliper, it's a similar situation. Fluid will flow from the fluid inlet through the upper crossover pipe and out of the nipple. It won't flow the other way. I guess if the upper crossover was significantly smaller that the lower one, the fluid would tend to go the other way, but you still have all the fluid that's above the fluid inlet that won't get flushed out. Either way, you need two nipples and they both need bleeding.

I had to have a similar conversation with the BMW 'technician' that tried to bullshit me when they only blead the inner nipples on my blue calipers. It probably works on most of their customers too, but they picked the wrong victim that day.

[TurboWeasel]

Thanks for that /\

Well my outboard bleed nipples look like they've never been touched. I guess I'll be doing it myself from now on!

Just need to find the correct paint colour for the calipers now, since the dealer destroyed the paint on one of them during the fluid change. It's definitely not Estoril Blue as the internet claims. It's a fair bit darker than that.

[Ennoch]

Originally Posted by ian332isport

As Ennoch has pointed out, you hardly ever get 4 (or more) piston calipers with a single bleed nipple. Some calipers have internal drillings to link the two halves of the caliper and some have an external link pipe like your old AP Racing calipers, but the principal is the same.

Even if you made a caliper with both upper and lower crossover pipes (and I think they do exist), you still wouldn't be able to bleed the outer half of the caliper properly. As with most things, the fluid is going to take the path of least resistance, so if you had a single bleed nipple on the inner half, the fluid is going to flow from the fluid inlet directly up to the nipple. You will get virtually no fluid flow through the lower crossover, back around through the upper crossover and finally out of the nipple.

If the single bleed nipple was on the outer half of the caliper, it's a similar situation. Fluid will flow from the fluid inlet through the upper crossover pipe and out of the nipple. It won't flow the other way. I guess if the upper crossover was significantly smaller that the lower one, the fluid would tend to go the other way, but you still have all the fluid that's above the fluid inlet that won't get flushed out. Either way, you need two nipples and they both need bleeding.

I had to have a similar conversation with the BMW 'technician' that tried to bullshit me when they only blead the inner nipples on my blue calipers. It probably works on most of their customers too, but they picked the wrong victim that day.

Ha, I had that conversation with the dealer the first time they did it as there was zero evidence the outer nipple had been touched. They dug their heels in and assured me they did it through the wheel. No matter, it was a poor bleed anyway so I redid it properly with better super Dot 4 fluid.

Yep, the mechanics of trying to fit a big brake caliper under a wheel necessitates two bleed nipples. The single nipple is common on motorbike and bike stuff but they also aren't quite so tight for space against the rim:

Originally Posted by TurboWeasel

Cheers for the detailed response and insight as always.

You're right, I just looked up the AP Racing CP 5200 calipers I used to have and they do indeed have 2 bleeders. It was 14 years ago, so foggy memory and all that.

That's the same calipers I've got. For a design that originated on race cars in the early eightees they've done well for themselves and although they're now metric rather than imperial, the hydraulic advantage is still there, and on things up to what's using the max 330mm x 28mm rotor dimensions, the caliper is also still more than stiff enough for the line pressures involved.

Originally Posted by TurboWeasel

Thanks for that /\

Well my outboard bleed nipples look like they've never been touched. I guess I'll be doing it myself from now on!

Just need to find the correct paint colour for the calipers now, since the dealer destroyed the paint on one of them during the fluid change. It's definitely not Estoril Blue as the internet claims. It's a fair bit darker than that.

It's definitely not EB2 as I've touched a few bits in (that BMW marked) using one of the pens I've got. On the caliper it looks too light despite the touch up looking too dark on the paintwork! I've seen suggestions of Monte Carlo or Le Mans being better matches but as the calipers will be manufactured by Brembo, I suspect that the paint will be theirs and theirs alone. Given the way it shows in comparison photos to test cards I think it's a darker base than the standard BMW blues but with a larger silver metallic particle in there. The bronzy gold on STI calipers seems similarly hard to pinpoint.

I do have a spare OSF caliper sat there which could be sent to Paints4U for a blend scan to be made if you wanted the best possible match. I've got a good relationship with them for things like that and they've been happy to scan things for me for free before whereas it's normally a chargeable service. I guess it's a question of how important the perfect match is given the area is on the rear. Careful masking should make it possible to sort it without having to touch the front face.