pedal to the floor after front disk replacement?

I just replaced the front pads on my wife's '89 740 GL. Used my standard
technique, pull the calipers off the disk, push the pistons back into
the cylinders with a c-clamp, new pads & reassemble. Master cyl.
reservoir is full.

Now, pedal goes to the floor & only slight braking on the front disks.
There was enough pressure to take up the slack on the pads, but no real
braking action. With help in the driver's seat, I found that there's no
significant fluid flow at the front calipers and no 'spurt' from the 2
lines leaving the rear of the master cylinder as the pedal is pushed.
The 2 lines on the front of the master cylinder spurt fluid normally
when the pedal is pressed & the fitting is cracked to bleed it.

Any thoughts out there on what could cause the back half of the master
cylinder to quit with just a change of brake pads?

Thanks,

Charlie

 

The pads are retracting too far. Recheck your work - I'm betting you will
find one caliper where the pads aren't even near the disk. Make sure you
have relubricated the slide pins and verify the piston in that loose caliper
doesn't try to retract itself.

Mike

 

Thanks, I'll pull them off tomorrow & try to extend the pistons a bit.
Is there something about the dual piston caliper that would prevent
fluid flow when you try to bleed the line at the caliper and also at the
master cylinder? The brake pedal goes to the floor now with only a
slight resistance & there's no flow from either line connected to the
back half of the master cyl when it's pressed.

Charlie

 

Right- it sounds like you (Charlie) know what you're doing, but just
in case you missed something simple, did you repeatedly pump the
brake pedal? After a few times (a few as in ten would be a lot), the
pedal should start pushing back. Did you try bleeding the brakes
over again, just in case you got air in their by accident? And make
sure you bleed them in the right order.

Hopefully you didn't kill the master cylinder. Sometimes the master
cylinder pistons can get used to only using up a bit of their travel
a little bit of crud and corrosion can build up inside the rest of
the cylinder walls. When the piston uses the full length of it's
travel, as in when pumping up the slack in newly replaced pads or
rotors, the corrosion damages the piston seals. (Does that make
sense the way I explain it?)

 

Theres a little junction box box where all the lines run into .Take the
plastic contact off and in side you will see a sliding pin "centre it "
and replace the connection and wire then all brakes should be equal unless
oil or air is in the junction .

 

I prepared to work on the car this afternoon & when I decided to check
the pedal one more time before tearing things apart, I had pressure.

I'm now confident the answer was hiding in the original symptoms: no
fluid flow from the back half of the master cylinder when the pedal was
pressed. I'm betting the the 1st long stroke (needed to take up the
'slack' of fully retracted the caliper pistons) moved the master cyl
piston into unexplored territory & it hung there. That was my 1st guess
when the problem presented itself, but repeated whacks on the housing
yesterday didn't help. Apparently, continuous return spring pressure,
temperature changes overnight, full moon, etc. must have conspired to
help the piston return to its normal resting location during the period
of inactivity. All is now well with the world (or at least with the Volvo).

Thanks to those who replied; I'll file those suggestions away for 'next
time. Hopefully, my results will be useful to someone. From now on, I'll
be changing one side, pumping back to normal pedal pressure, then
changing the other side.

Charlie

 

And thanks for the feedback. It will help us tune our responses next time!

Mike