At 09:59 AM 6/3/03 +0100, Telewest \(PH\) wrote:
>....
>Interesting. If there is no vent to the atmosphere then how?
Most modern brake systems have a sealed reservoir on the master
cylinder. This is done with a flexible elastomer diaphram in the top of
the reservoir, commonly captured under the lip of the cover similar to
installation of a gasket. The cover itself is vented above the diaphram,
so as the fluid level fluctuates the diaphram can move. Air is allowed to
breath in the reservoir, but only above the diaphram where it is isolated
from the fluid.
If the fluid should happen to be overfilled near the very top of the
reservoir, and the diaphram and cover then installed obove the fluid
leaving a very small breathing space, it would be possible for the fluid to
expand enough with heating to fill the entire reservoir forcing the
diaphram up against the cover to expell all of the air and stop the
venting. Then additional thermal expansion could indeed apply pressure to
the system to cause the brakes to drag, which would cause additional
heating and likely lead to total lockup. I suppose it may also be possible
for the vent hole in the cover to become plugged.
To check fluid level, remove the cover and the diaphram. The reservoir
should (probably) have a fill level line for the optimum fluid level
(although I have not personally seen every unit ever made). As brake pads
wear, the caliper pistons move forward in the cylinders taking in fluid
which will lower the fluid lever in teh reservoir. With a maintenance
check, someone may add fluid to the proper fill line.
Then the problem may occur if someone installs new brake pads without
removing fluid from the reservoir. As the caliper pistons are pushed back
into the cylinders to accomodate the new full thickness brake pads, the
fluid is forced back into the reservoir. If no fluid is removed from the
reservoir it can end up being overfilled, possible to the absolute top with
no breathing space above the diaphram. In that case, even a little thermal
expansion could make the brakes lock up.
If the Jeep is still having the same problem, check the fluid level in the
reservoir, and check to see that the vent hole in the cover is still clear
and breathing. You could also try leaving the reservoir cover loose for a
while so that it could not possibly build up pressure in the reservoir, and
see if that bannishes the gremlin. Be prepared for a possible fluid spill.
Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com
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