Campbell,
I read your post about check valves in disc brake systems and have to
disagree. A check valve is suicide in a disc brake hydraulic system. The
pads constantly drag against the rotor (barring any kind of "quick take up"
master cylinder) just slightly. This creates heat. The heat causes the
fluid to expand. If the fluid has nowhere to go (i.e. back into the master
cylinder), it begins to apply the piston (simple expansion of the fluid due
to heat). This creates more heat. Eventually you will seize up solid (and
end up on your head). This is the whole purpose behind the compensating
port in a disc brake master cylinder (that is the second hole in the master
cylinder.....the one that squirts up when you depress the brake without the
m/c cover on). If the m/c is adjusted incorrectly, the compensating port is
covered by the cup seal on the m/c piston and the front brakes (or any disc
brake system) will lock up. The actual purpose of the check valve
(otherwise known as a residual pressure valve) is to provide adequate
pressure to keep the lips of the drum brake wheel cylinders firmly planted
against the wall of the cylinder. This stops air from being sucked in (and
the resulting spongy pedal). We (I'm a licensed Automotive tech) have been
always told to remove any check valves from jobber master cylinders that
might accidentally come with them. In fact, modern cars with disc/ drum
combinations have the check valve (for the drum circuit) in the combination
valve (pressure differential valve, indicator, and residual pressure valves
all in one) or the ABS EBCM, not the master cylinder.
As far as Thomas' problem with his brakes, he needs to differentiate
between too much lever travel and spongy lever. If the only things that
have been changed are the hoses, then logically air in the system is the
only likely cuprit (for sponginess). My suggestion is to remove the lever
assy from the handlebar and slowly rock it in all directions (cover on or
off) to see if there is any air hidden in the master cylinder area.
Reinstall the lever and then begin the bleeding process right at the lever.
Loosen the brake hose fitting on the m/c while holding some pressure on the
lever. The stream of fluid must be clear and have no bubbles at all. Next
repeat this on the hose fitting on either caliper. Then finally open the
bleeder on that caliper. Then repeat this for the other caliper. The lever
should fade away as the fitting is loosened, and the fitting retightened
before the hand pressure is released from the lever. Mitivac's work well,
but you have to ensure a constant source of clean fluid at the m/c and any
air ingested by the system only makes the process longer. You can also be
fooled, as well, by the stream of bubbles coming out from the bleeder.
Remove the bleeder and wrap it (in the correct direction) with teflon tape.
Reinstall and continue bleeding. The teflon minimizes the possibility of
being fooled by false air. If he's concerned that the air gap (between pad
and rotor) is creating difficulties, use the very shims (feeler gauges) used
to measure it (or better yet use some small block Chevrolet starter shims)
as spacers to take up the air gap and then rebleed the system in a logical
and consistant manner.
Don Seel.
.............................................
To unsubscribe go to http://www.sayegh.org/unsubscribe.htm
.............................................
