Civic Brake Pedal Sinking

If it is actually sinking as opposed to just being spongy and there are no
leaks then I would think the master cylinder is bad again.

CaptainKrunch

 

Vehicle - 1994 Civic GLi

I recently had a problem where the brake pedal would begin sinking to the
floor on a hot day when depressed. It was suggested that I replace the
brake master cylinder. The master cylinder was replaced with a brand new
Honda master cylinder but I am now finding that the brake pedal when
depressed sinks very slowly on any day, to the point that a clunking sound
is created when it reaches the furthest point. The brakes work fine but the
pedal, when depressed for over 20 seconds while the vehicle is idling, sinks
to the floor by 20mm.

I also conducted a brake booster test as per the Haynes service manual
instructions. The tests revealed that the booster is functioning correctly.
The brake pedal, when depressed while the engine is off, gets firmer with
every press. However, it does not get to the point where there is no play
at all. It gets firmer but there is still 10mm of play when depressed
firmly. Is this normal for a Civic? There was no mention of this in the
service manual.

The brake fluid reservoir has been carefully inspected and there is no sign
of fluid loss.

Any suggestions?

 

It doesn't sound extremely abnormal but there could be air or moisture
in the lines. Flushing and bleeding them is certainly a worthwhile
next step.

If that doesn't solve the problem, and it doesn't seem unsafe, I would
drive it into the warmer weather. If the problem gets worse, it could
be that the new MC is bad. If it never gets worse, it is probably
normal.

 

You might want to trace what the clunk is and where it comes from but
contrary to what others have told you, if the 20mm is measured at the brake
pedal foot pad it's normal. I've checked this carefully myself because I'd
noticed the very hot day, A/C on, pedal sinks towards the floor on a 6
year-old Integra. I first noticed it in traffic when the car was at a
standstill and I was holding it on the brake. The brakes worked normally
otherwise, IOW no soft pedal on normal braking.

I was still worried about the sinking pedal when stopped with very high
underhood temperature so when I got a new Integra I did a test to see how
it behaved. Yes it did the same thing but not to the same extent - IOW
even with the new car, under the same conditions, the pedal would sink but
not quite as far. With the old car it would go a little more than halfway
to the floor; with the new one it would go about one third the way to the
floor - easily 20mm measured at the pedal foot pad.

In fact I'd suggest that some of the people who have told you that you have
a problem actually do the check and especially with high underhood temps.
I don't have a non-Honda to check against but this seems to be an
idiosynchrasy of the Honda brakes to do with the hot master cylinder, hot
brake fluid and hot brake lines. It gets worse with age but unless the
pedal goes all the way to the floor or the pedal sinks under normal
braking, I don't think there's anything to be concerned about.

It depends on what you mean by "play" and where you measure the 10mm. If
you mean a total of 10mm of pedal travel, counting mechanical play before
the pedal pushrod contacts the piston, *plus* any hydraulic "take-up" it
seems normal to me. IOW from its rest position, you press the pedal and
your foot travels for 10mm then that's OK; obviously there has to be some
play and 1cm is not very much at all. If you want to be sure of this go
check out a car in the showroom.

Rgds, George Macdonald

"Just because they're paranoid doesn't mean you're not psychotic" - Who, me??

 

Yeah I agree, my '90 Integra had the sinking brake pedal syndrome with
the a/c on. It sinks a little extra and it's something weird about
Honda.

If it goes to the floor (EVER !!) then your master cylinder is shot,
or the lines are in very serious need of bleeding. Do not drive it
this way; get it serviced but quick.

JM