wearing out the master cylinder

I just had my brake fluid flushed and replaced (along with new brake pads in
front and the rotors machined).

Afterwards I noticed -- when at a complete stop -- my brake pedal sinking
slowly to the floor. I have to pump it once in order for it to stay firm
while applying pressure.

I told the service advisor that the brakes feel as though they weren't bled.
Instead of following that route, the advisor advised me that there's a risk
in permanently damaging the brake master cylinder after one too many fluid
replacements. 'Says the piston ends up in a slightly different spot after
each job and gets worn in that spot. So, after many service jobs the thing's
worn all the way around.

Does this sound reasonable?

 

in

I think people are being charged for changing brake fluid that does not need
to be done. I have never changed brake fluid. If you have ever rebuilt a
brake system you will find slight sludge around the seals in the wheel
cylinders, calipers and in the system. So if you really want to do a good
change you would have to remove all components, flush lines, replace all
seals, combination valve, master cylinder, etc. By not changing these you
are running the chance of loosening particles etc and making things worse.
You are better off by not changing. They will start wanting to sell you a
freon change soon. Any good honda mechanic will not change brake fluid on
his car if the brakes are working fine, why should you?

 

do you feel the same about oil, tranny fluid, and coolant too?

brake fluid has a service life. it collects water and crap from the brake
system, and does need to be flushed periodically.

 

Yes I've seen this effect: in traffic sitting stationary with foot on the
brake pedal, it sinks slowly towards the floor. The effect is much worse
when it's hot, the A/C is on and underhood temps are very high. When I go
to check the system with it cooled down a bit (still pretty warm though),
everything checks out as normal - in particular, with engine off and vacuum
boost bled off, the pedal feels as solid as a rock. I also don't feel that
sinking under normal braking conditions, with the car moving.

After noticing this, I checked the next new Honda I bought for the effect
and it is definitely still there, even on a new car... just not so
pronounced. I believe that it's partly due to the high temps expanding the
master cylinder a bit and also allowing slightly more expansion of the
rubber brake hoses under high pressure. There might also be a quirk of the
vacuum boost system at high temps and high vacuum (engine idling) which is
contributing here. With an older car, add in a bit of wear in the master
cylinder and its seals and it gets worse.

It depends somewhat on how they bled the system - if with a pressurized
reservoir fitting then no it's not reasonable to me. If they bled it by
pedal pumping it's been remarked here that the seals can be slightly
damaged by travelling in a part of the master cylinder they normally never
reach and which may have some corrosion build up from moisture in the brake
fluid.

My advice: you have to figure how bad the effect is. If the pedal goes all
the way to the floor or if you feel it's starting to affect actual braking
or if the pedal sinks, or feels excessively spongy, when you do a test in
less severe temps with no boost applied, it's time to replace the master
cylinder.

Rgds, George Macdonald

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

 

due to federal safety standards brake fluid is made with high standards,
under normal use the fluid will last many miles. When the pads or shoes wear
out then change the fluid and rebuild wheel cylinders or calipers or replace
them.When you get a fluid change the fluid is used from drums or pressure
containers. Guess what, the pressure container is pressured with a shop air
compressor that contains mositure in most shops.Engine oil, trans, etc
should be changed. Most of the folks I know that changed their fluid when
the brakes were working fine have had problems.Every time a new airforce
plane comes on line the tech orders call for nitrogen to be used to inflate
the tires this is supposed to stop wheel corosion.. After a short time this
practice is given up, because it is overkill.