Ok, most likely it's up in the front caliper area. That's not an absolute
cast in stone sure thing, but it's a pretty good bet.
The leverage factors make tiny little motions in the caliper immense
at the brake pedal. Real quick rundown with some rough numbers:
Master cylinder bore is about 3/4". That's about .44 square inches
surface area. The caliper piston is about 2-1/2", so that's about
4.9 inches surface area. That's a factor of 10 increase, and there
are four of those caliper pistons.
You've got about 10" of brake pedal travel, and that gives about
2" of piston travel in the master cylinder. If you had just one
piston, it would move only 2/10" , with four pistons, it's a mere
.05 inches! That's with the brake pedal going all the way to
the floor.
So, if the piston seal is sticking on a caliper piston, or a
caliper is a little lose and cocking, or something is flexing,
it would greatly affect the brake pedal under your foot.
What you may want to do next lunch is get a friend to go
out there and be pumping the brake pedal. Look at each
front caliper and observe the motion of things. You really
don't want to be seeing anything move. The pads should
only faintly flex in, and the caliper shouldn't move at all,
nor should the rotor. If you can pull the wheels, look in
and see how the piston seems to be moving against the
pads as well. That's hard to do, but will tell you if you've
got a sticking piston seal that's pulling the piston back.
As long as you're doing this, also go listen at the rear
of the car. If you hear a long scraping sound or the
sound of a spring popping along as it's being stretched,
you've probably got excessive travel happening at
that side of the rear.
I'd also consider a re-bleed. Brake fluid is cheap, and
as I recall you weren't too pleased with the bleeding
results you got. A small bit of air can be compressed
into nothingness, giving a soft squishy pedal that goes
firm when the air bubble is squashed away. It's a
different feel then excessive travel, but it's something
to be aware of.
>>> Richard B Gosling <Gosling_Richard_B@perkins.com> 10/13 8:26 AM >>>
Nolan,
>From your description it doesn't sound like my M/C - I always get lots of
travel (down to the floor) on the first push of the pedal, firm pedal on the
second push, regardless of how fast or slow each push is. I have
experimented, pressing with different combinations of speed, to check
repeatability of this. This also agrees with Eric's advice, that if I really
hit the pedal hard, I should get a firm pedal first push, if it was an M/C
problem. It definitely feels like I get a lot of movement, then the brakes
start to work, not the brakes start to work with the pedal high up but the
pedal then sinks - although I accept that impressions can be deceptive!
I bled the brakes yesterday - no improvement. I checked the rear brakes like
you said, Nolan - pump up footbrake so pedal is firm, while applying footbrake
then apply handbrake, release footbrake, wait, then try footbrake again. It
had as much travel as ever. This, from what I understand you saying, would
rule out the rear brakes as the problem area.
I checked the front brakes lunchtime today. I only had time to do one side.
The pads were a very tight fit in the calipers - in fact I remember having to
file a bit of the metal backing plate off to get them in in the first place.
I took them back out, ground some more metal off the top and bottom edges of
the backing plate, and replaced them. They now move freely. However, my
brakes are still just as bad as ever. As I said, I have only done this one
side due to lack of time - Monday lunchtime I will do the other side, and see
if having both sides done helps at all. You may all realise by now that all
of Daffy's repair and maintenance occurs in 3/4 hour sessions during my lunch
break - as a husband, and father of a 1-year-old, it is the only time I can
find!
Richard and the unstoppable Daffy
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