The two bleeders Options

[emerygt350]

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.

[davep]

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.

[Superhawk996]

Dual bleed nipples of the 914 rear have nothing to do with handedness, the parking brake lever defines that.

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.

[davep]

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 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.