Chris stay cool, everyone has had some success with all systems you are not alone with the vac and you can bet someone loves it and some hate it as they do pressure systems, and hey I want you doing your 'moderator thing' so earn your keep!!
Frank, I am not sure about positive pressure. If the positive pressure was a constant (parked or otherwise) the brakes would bind all the time creating drag? When the pedal is released the pads retract, part of this is due to the rubbers deforming and the rest as the piston returns to rest i.e. pulling back on the fluid, as you know you never tighten the nipple on an up stroke for this very reason, and the amount of air needed to make a pedal spongy is very small indeed so.....................?
There is a possibility that the air is getting in through a very tiny leak but not big enough to leak fluid unless, the brake pedal was pushed and perhaps held over a period of time, perhaps start engine, wedge pedal down and switch off and leave overnight?
Awkward this one mate,
regards
Dave
I've been reading this thread with interest and although I'm not up to date on ABS and have no idea what role "modules" play and "electronics" in a braking system, IMO (as Chris has intimated) the thread has gone awry on basic physics and the basic function of braking systems.
A "positive" pressure as described by Frank, that is to say the minute gravitational difference in height between the MC and other components of the system could never cause "bind all the time creating drag" as suggested above. AFAIK there are no springs in the calipers that would induce "the pads [to] retract, part of this is due to the rubbers deforming and the rest as the piston returns to rest i.e. pulling back on the fluid".
There is no "pull back on the fluid" from the calipers, the pressurized fluid simply returns to non-pressurized fluid (with the exception of the minute gravitational pull mentioned by Frank).
Some of the posts on here have, IMO, tried to make the simple process of pressurizing fluid in a pipe system, way more complicated than it is.
There is almost no travel of fluid in the pipes when the system is in function. It is pressurized and it is relaxed. The "movement" at the pads is microscopic. A disc (or rotor) in good shape and assuming no other factors such as worn wheel bearings, will not push the pads back, the pads simply become relaxed and stay where they are, lightly brushing the face of the disc.
Tightening the nipple on the down-stroke is simply to ensure no air is drawn into the system before tightening.
My experience is that when a bleed nipple is opened (assuming all in clean and functioning) the fluid will drain under gravity but it is very slow and hence the use of the "up-down missus", vacuum or pressure bleed tools to assist.
In the old-days, I used to run a plastic tube from the bleed nipple back to the MC, but that was when I was permanently broke, and the thought of pissing perfectly good brake fluid down the drain was undesirable. Of course, I'm more educated now and realize it's quite a good idea to change fluid from time to time, so running fresh fluid through the system is not such a bad idea.
BAT's intermittent "pedal to the floor" syndrome is a bit of a mystery, sometimes MC rubbers get deformed a bit and allow fluid past the seals, but having replaced the MC, one would presume that sort of problem resolved.
I'm also not as confident in replacement parts these days, BAT's rear compensator valve replacement is a good example.
On that note, I lifted my suspension by 110mm, the fitter told me he'd adjusted the compensator and I could see that it had been moved.
I get lock-up on the front wheels on full braking running 305/70/16 tyres and at each test, I get a pass reading on braking balance and efficiency (including the rear) where on my 80, I have drum brakes and no ABS.
Just my pennyworth.
Anyway, best of luck getting to the bottom of this BAT, hope all turns out well.
