Question re: AP racing brake fluid reservoir

[GeorgeK]

I have just bought a dual MC pedal cluster for my lwt Carrera 3.o (in prearation for the 930 brake changeover). With the cluster came a fluid reservoir, very nice and much cleaner than the tired one in the car.
In the bottle is a concertina type of rubber "bellow" which serves a purpose according to AP racing.

Does anyone know what is its use? which way does it go in the reservoir?

From APracing's site:

"Rubber Diaphragm (Bellows) CP2293-48

Minimise the entry of moisture and dirt and helps prevent spillage. For use with reservoir cap 4325-148 on the following reservoir assemblies. All CP2293 series & CP4623-7 & -8.
N.B. Use of bellows may restrict effective volume of reservoirs."

[911pcars]

As was described by AP:
"Minimise the entry of moisture and dirt and helps prevent spillage."

A flexible bellows usually goes into the reservoir so that it expands downward as the fluid level goes down. A bellows separates ambient, moisture-laden air from the fluid. A sealed, rigid separator would create a vacuum in the reservoir and prevent fluid from filling the system.

Good to see run-of-the-mill GM and Ford technology used in a racing reservoir. ATE should have use something similar.

Sherwood

[GeorgeK]

Hi Sherwod,
Makes sense so far, but how does it prevent moisture entry, and which way does it go in the bottle? There is a small hole at one end of the bellow; top or bottom?

[911pcars]

George,
I haven't seen an AP BF reservoir bellows so I can't comment on it's construction. However, as seen on many numerous BF reservoirs, the bellows expands downwards toward the BF level. The bellows is sandwiched between the reservoir and the cap, and if sealed (no holes), no air can migrate past the bellows when the cap is closed (threaded or clamped). Since it's flexible, the bellows will follow the BF level as it fills the system (from brake pad wear). An analogy would be like your eardrum.

Got a pic?

Hope this helps,
Sherwood

[GeorgeK]

Sherwood,
No pics, but the description of the principle makes sense to me. If the bellow makes a seal with the cap, and the hole in the bellow communicates through the cap with the outside, then the descent of the fluid from pad wear will expand the bellow *witout* letting air enter the same space as the fluid.
Thanks for the insight

[911pcars]

George,
I guess we're okay except for the "hole in the bellows". The bellows doesn't need a hole for venting as its flexibility allows the air volume below the bellows to remain the same as the fluid goes down. It is the vented air volume above the bellows that expands as the bellows descends.

Another analogy. Think accordion bellows.

Hope this isn't more confusing.

Sherwood

[Bill Verburg]

Think of it as a condom that allows expansion and contraction of the fluid, but prevents(or at least reduces) contamination of it.

[911pcars]

Everyone have a mental picture of Bill's analogy? Good.

Sherwood

[Bill Verburg]

glad to make it easily understood