I just popped on my newly remastered rear brake calipers from PMB and was wondering if there was a point to the lower bleeder? I only ever bothered with the top bleeder before but now that it's bone dry I was wondering if that might be why the second bleeder exists? I will be bleeding the brakes tomorrow morning.

Good question. Curious, too. Dual bleeders on later cars (74 on?).

Dual bleeders facilitate the part being used on either side without need for handed parts.

Lower bleeder is useless. Always bleed from the top bleeder since air rises to the top.

On one car I had a harder time than usual getting a firm pedal. I shot some fluid up through the bottom bleeder with a syringe and tubing and voila….

Dual bleed nipples of the 914 rear have nothing to do with handedness, the parking brake lever defines that.
However, I have found that when doing an annual fluid bleed it is often better to open both bleed nipples to fully drain the calipers before refilling. Forty years ago I had to start rebuilding the calipers to keep them fully functional; there was no PMB to send calipers to and there were no (!) parts available in any case. Taking the pistons out was a real eye opener since the outer pistons were always filled with more crap than the inners. The reason for this is simple. The fluid inlet is on the inner side of the caliper as are the bleed nipples. When you do a quick flush and bleed the fluid enters the inlet and heads straight for the open nipple. This does not flush the fluid inside the pistons, particularly the 914 rears with the adjuster mechanism inside, and it certainly does not do much of a job in circulating to the outer half of the caliper. The result is that old fluid is retained inside the pistons and most of the outer caliper half with a quick flush and bleed. Hence my method was to drain the calipers first, then do a complete refill and bleed. Every few years I would disassemble the calipers to thoroughly clean inside to keep them operating efficiently.

I do hydraulic system flushing all the time...
On almost all cars the entrance into the caliper and the bleeder is on the upper side of the caliper and a flush does not get ANY of the old fluid in the caliper. The fluid in the caliper is what goes bad first!
What I do, on normal appliance cars, is to apply my vacuum bleeder to the bleeder bore and push the piston back into the caliper clearing most of the bad fluid out.
After I get one collapsed, I'll feather the brake pedal, going no farther than normal pedal travel, and push the piston/pistons back out.
Do this twice (time consuming) and you get 95% of the old fluid out of the caliper.
The hardest ones to do this with are fixed calipers with opposing pistons like what we have on Porsches. Datsun Zs have the same set up but they don't have the two bleeders.
Having two bleeders makes this far easier to do.

Thanks to Davep for showing me a trick that I didn't even think about.
I've been doing this crap a long time and seldom do I have a "Eureka" moment any more.
Davep gets a for that little tidbit

Now, I'll return the favor...

One thing I do after I'm finished with a flush job is to blow the bleeders out with a needle tipped blow gun. Brake fluid is hydroscopic and that little bit of fluid that get left in the bleeder draws moisture into the bleeder and IMO is the reason bleeders rust into their bores.
Don't breathe in the cloud of brake fluid that this will produce. Brake fluid stays in the human body forever and inhaling it is bad.
I'll put a tiny little blob of silicone grease in the rubber cap (most cars have this) to seal the bleeder from the outside atmosphere after I blow the bleeders out.

Thank you for clarifying this, the dual bleeders have been a mystery for me and I am sure many others.

Disagree. You have to look at this from a manufacturing perspective.

Take a look at the inboard side casting, it is symmetric. The casting is not handed until the machining operations are done for the handbrake.

Having lived and worked in a manufacturing environment, my best educated guess is that the piston bore and the bleed screws were machined before the park brake bore.

This allows the casting to remain unhanded for easier inventory management. Since ATE was suppling these calipers to other OEMs there would be an advantage to not having handed calipers until as late in the machining process as possible.

In this day and age, no one would do two matching operations and provide two bleed screws. It amounts to giving away money. But back in the 70s before computer inventory management and just in time delivery of parts there would be a great deal of value in minimizing handed parts.

I'm in agreement with this. They made one casting each for the inside parts of both front and rear calipers to reduce tooling costs.

Now, with the information that Davep provided regarding servicing, this should have been included with ALL caliper designs.
Not just Porsche ones!

My Dad once had a 964 S4 that had been heavily modified. It had the Porsche "cup" suspension on it and the "Big Red" brakes. Whoever installed these did not get a matching "kit" as it had two front left calipers and those puppies only had one bleeder.
The right-side bleeder was pointing down which made it impossible to bleed unless you knocked it loose, stuck a piece of wood between the pads. and held the caliper bleeder up for the bleeding process.
That worked fine but it was a bit of a PIA to do.
Had they had the dual bleeder set up it wouldn't have been a problem...

Shoot, wish I had read Dave's comment earlier. Finished bleeding and drove her around for the first time this season. Nice to have rear brakes finally. And a functional emergency brake. I had seized pistons and obviously couldnt set the venting. PMB to the rescue. Now I am tempted to go back through the fronts and try the little drain trick to see what the fluid looks like in there.

Car ran great after sitting for 4 months. Did a little oil and filter change, all ready for summer!

I also worked in manufacturing (still do actually) mostly on the management side, and had to develop the procedures for many manufacturing tasks.
I know of no other customer using these calipers; only the 914/4. The 914 was a high cost model for Porsche, and they were pinching pennies everywhere. They were already using computers in 1970 as well. But for sure, they did not use just in time delivery of components.
To machine the caliper inner half I would start by boring and facing the back side of the mounting ears; not a critical operation in itself, but it does set up the casting to be securely fixtured for the piston bore operations that are a critical step. If the part passes inspection after the piston bore is done then it can go on for other operations. I would face the mating surface, the pad box and the other side of the mounting ears next. The adjuster bore is another semi-critical operation so that the parts from either side mesh correctly. Up to this point it is not handed, but the pad pin bores and the the parking brake operations will define the handedness. The last operations I would do are all the fluid points and the fastener points.
The piston, adjuster mechanism and the parking brake mechanism are all interconnected parts that need fairly precise machining to operate correctly. The fluid points are really independent of each other as well as of the piston operation. The mating fasteners and pad pins are more critical for positioning (alignment) than the fluid points.

I think the one thing we can agree on is that these calipers with two bleed screws on the same side of the casting are a bit of an anomaly among the brake world.

I’m certainly not going to claim that I know the machine operations with certainty. Despite the logical machine operation you’ve laid out, I’ve also seen plenty of instances where machine operations are dictated by the order of machines on the line or the direction that machine slides (axis) are oriented rather than the order of operations we might do if we had flexibility to do them in a logical order.

It certainly would be entertaining to know the details of exactly what ATE did and why they were willing to give away an extra bleed screw and the machine operations that produced it.

Ferrari 308 uses the same rear calipers. I think the double bleed screw is to accommodate use on it.

So I’ve never actually looked at the Ferrari application. It seems they do use the caliper with the handbrake lever on the top instead of on the bottom like the 914. So that would need the bleed screw flipped 180.

The rear casting also appears to be the same at a glance but I certainly don’t have parts in hand.

I believe the Ferrari really used the /6 GT vented rear caliper. But again I have always been under the impression that the 914/6 back casting was common between the 914/4 and the 914/6 and that both the Ferrari 308 & 914/6 GT just added a spacer between the inner and outer casting to accommodate the wider vented rotor. Yes, the /6 has a larger 38 mm piston and a the stock rotor OD is a couple mm larger and a mm or so thicker. This would be an easily accommodated within a common casting.

To me that seems like more rationale why ATE might have done generic machining like bleed screws early on before the inner casting final use was known and more of the application specific machine work was done.

Would be interesting to have both applications in hand. I think PMB might have insight but suspect this topic has been beat to death in the past though the Google magic isn’t finding a thread with a 100% definitive answer on the dual bleed screws origin. Here’s a video from rebuild of Ian’s 308 calipers by PMB.


https://www.youtube.com/watch?v=oUvYRPe2bDw

No, the Ferrari caliper is completely different.
The Ferrari caliper is similar to the 914/6 rear caliper, physically larger since it uses the same brake pads as the 914 front calipers not the tiny 914/4 rear pads. The Ferrari caliper has the handbrake lever on the opposite side and hence the single bleeder is on the opposite side also. Further, the Ferrari caliper has two identical pins for the pads, not a large and small combination.
I was looking around for some 914/6 rear calipers some time ago and came across a Ferrari caliper for sale that was converted to Porsche style by adding a second bleeder. I had the shop convert a 914/6 rear caliper to have dual bleeders so that I could have a matching pair; well not really, the one has 2 small pins and the other has the large/small combo.

Here are my 308 calipers. I had a machinist add the second bleeders so they are on top when mounted on my 914/6 conversion. Fresh from restoration at PMB last month.


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Nice

Any chance you have a /4 rear that you could post side by side? Long shot I know but can’t find that rear inboard casting comparison anywhere online thus far.

I just can’t imagine ATE having a unique casting for /6 and Ferrari 308 given the low volumes. Could be wrong but would love to see the side by side comparison.

Nope - I’m wrong on a casting common to all three applications. You can see the /4 casting has a different formation than the Ferrari rear inboard casting.

The other thing that is implied is that Krieger’s calipers had to be drilled to the /6 orientation meaning that the 308s weren’t dual bleed screw.

The mystery continues as to what would prompt ATE to have two bleed screws on the /4. I’m still thinking someone else was using the /4 casting for another OEM. That will take some digging. I’ll see what I can find on ATE classic site.

/4 photo from Pelican
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Here’s a previous discussion comparing /6 and 308 rear calipers in gruesome detail. Nothing about /4s, unfortunately.
http://www.914world.com/bbs2/index.php?sho...&hl=Ferrari

No can do. The 308 calipers are back on my car. They look like they are ~ 25% larger than the 4 cylinder ones

And some casting variations with mine. @Superhawk996 . I bought these from a Ferrari repair shop. They told me they were off two different cars. I didn't care since they were $100

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Thank you for the alternate views.

Nice score for $100. Lucky the didn’t call the cops after you stole them like that!

The ONLY applications that use this type of rear caliper are:

914-4
-914-6 - use a larger version of the 70-72 914-4 piston guts along with the M10 adjuster threads....a unicorn exclusive to the 914-6 caliper
Ferrari 308 - uses the same piston guts and M10 adjusters as the Late 914-4 caliper.
Alfa (38mm, but different parking brake lever configuration and a few other things)..use the same piston guts as the Ferrari 308, but the external threads on the outer adjuster are M10x1.25 rather than M8x1.25 like the 914 (4 and 6) and Ferrari calipers, and the inner setup is different as well.

I've also seen one instance of these being used on a BMW....not sure if OE or not, can't remember the model, but it was something special.

The inboard castings are a bit different between the early and late 914-4...the area where you insert the clips to retain the pad pins has a bit more relief on one than the other...maybe just an updated tool, who knows.