hydro brakes: reservoir orientation on bent bikes? Vert=bad?

I ride mountain bikes a lot and am very familiar with hydraulic disc
brakes. Most current systems are "open", or have a fluid expansion
reservoir at the master piston/cylinder (brake lever). If a bubble
appears in the system, it can travel "uphill" through the system and
escape into the reservoir. Bubbles in the system (other than in the
reservoir) cause mushy lever feel since the gas pocket is
compressible. The open system takes care of this problem if the
bubble is able to escape to the reservoir.

Many bents, however, use drop-grip-style bars which place the master
cylinder in a vertical or sloped orientation. The hydraulic line arcs
up to the handlebar and then descents into the brake lever (i.e. lever
blade points down), effectively disallowing bubbles from migrating all
the way to the reservoir. This can be seen on some Challenge Bikes
models. Trikes which use hydro discs (Greenspeed) invert the master
cyclinder (lever blade points up) so the reservoir is still at the
highest point.

Magura's rim brakes (HS 33, etc) employ a closed system (no reservoir)
and work under the assumption that no bubbles will ever form and the
brake fluid never will get hot enough to need to expand significantly,
and even if it does, the volume of the system can be adjusted via a
screw-piston.

My question is, does anyone here run an open hydraulic system on their
bent with the reservoir not at the system's highest point, and if so,
what are the results? Frequent need to bleed? Mushy brakes? Do you
remove the levers periodically and store them to allow bubbles to
migrate up? Many disc-brake equipped bents employ mechanical discs.
Is it for this reason or just because they are less finicky for the
average user (I have both on my mountain bikes)?

Thanks gentle bent folk.

tscheezy

tscheezy wrote:

My question is, does anyone here run an open hydraulic system on their
bent with the reservoir not at the system's highest point, and if so,
what are the results? Frequent need to bleed? Mushy brakes?

I have a Hope M4 on the front of my Speedmachine and a Hope Mini on the
rear, with the brake lines running upwards before being routed along the
tiller. No problems with feel or bleeding once they were correctly set up.
The rear one is a bit mushy, probably because I was too idle to get it
completely bled when setting it up.

Do you
remove the levers periodically and store them to allow bubbles to
migrate up?

No.

Many disc-brake equipped bents employ mechanical discs.
Is it for this reason or just because they are less finicky for the
average user (I have both on my mountain bikes)?

Probably because they're about half the price of hydraulics...

--

Dave Larrington - http://www.legslarry.beerdrinkers.co.uk/
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