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240 brake pedal to the floor...a few pumps and its fine. new master?

BRAKES 101:
on a NEW master cylinder you can push the seals hard enough at the bottom
of the "stroke" to slightly deform them - as they come back past the compensating
port in the master cylinder the seal gets a small scar in it..leads to premature failure

in an OLD master cylinder there is an accumulation of seal bits / rust - corrosion and
a *stuff ridge* of grot that builds up at the end of the *normal stroke* - if you "bottom
out the pedal you push the seals thru this "wall of crap" and they get cut / scarred /
sliced / ripped ....leading to failure...yearly flushing of the fluid (as in a COMPLETE
FLUID CHANGE OUT) will help stop the crud / corrosion ring of crud and gives you
the best chance at long life in the cylinder...
often unlearned / unskilled mechanics will "force the pistons back" whilst replacing
pads..this runs ALL of the accumulated mung in the system BACKWARDS THRU
the master cylinder leading to premature failure (if you have an ABS system onboard
you run the risk of forcing all the crap into the control valves in the ABS "block" and
effectively destroying THAT high $$$$ piece of gear...

I have yet to have any problems that source...
 
"... often unlearned / unskilled mechanics will "force the pistons back" whilst replacing pads."

Sorry for a lunkhead question, but don't you have to force the pistons back to fit the new pads? This is a bit of a threadjack, but I always try to pick up tips from the masters when I can, and hope I've not been doing it wrong all these years.
 
If you open up the bleeder screws when you push back the pistons most of the old fluid will come out of them (do work up a way to contain it - makes life much easier).
 
If you open up the bleeder screws when you push back the pistons most of the old fluid will come out of them (do work up a way to contain it - makes life much easier).

^V^ (and a BIG PLUS 1)
*BINGO* :-D

the added plus of this "way of doing it" is you get a bit fresh fluid every time when
you're done as you "top of" the reservoir! :nod:
 
I think the fluid displaced by one pot in one freshened 240 brake pad's worth of travel is roughly* equivalent to 4 feet of 3/16 brake line. With two pots per line on the fronts, a just-in-time pad replacement on the left-front wheel might send gunk from the calipers all the way home.

* roughly sez the pots are 1" dia, 1/2" travel, and the 3/16" line is 1/8" ID.
 
thanks for all the information!

Thanks for all of the information to help me resolve this issue.

I will be picking up a new (not remane'd) ATE master cylinder, reservoir and seals this afternoon from the European auto parts guy close to the office, as well as 2 liters of DOT4 brake fluid (I'll refer to the Bentley to make sure it is DOT4...I seem to recall it being that but I've made too many mistakes to trust myself at this point.)

This weekend I will be repairing this mishap in the following manner. Let me know if this is overkill:

1) Jack car, remove wheels, test all bleed screws for operation and add clear hose 3/8 rubber hose to each bleed screw. Prepare my large fluid collection pan and nitrile gloves (hopefully one pair will be enough).

2) Pull the pads and inspect..look for signs of leaking at the piston seals. Look at the bias valve to see if there is any leaks from that and trace all of the brake lines as best I can to see if they are all dry and not scratched, bent or kinked, etc, etc. Assuming all is OK since the front pads were don3 in March and the rear pads were done 2-3 weeks ago now...and all of the calipers are dry and not damp/wet....

3) Add brake fluid to the reservoir and bleed entire system with old master cylinder on there to get all of the old stuff out completely and have it with clean fluid. Bleed sequence will be as shown in the Bentley manual. Measure amount of fluig that went in, and that was bled out to assure all new fluid int he system. Discard old fluid.

4) Use my trusty syringe to remove all of the brake fluid from the master cylinder reservoir. Discard fluid.

5) Using a small pan and rags, remove the brake lines from the master, and the master from the booster.

6) Install the new master, connect all brake line fittings.

7) Bleed entire system (again) with fresh fluid. Measure amount that went into system and amount bled to assure all new fluid in this now second bleeding.

8) Top off reservoir as needed.

9) Start car..and test drive.
 
I think the fluid displaced by one pot in one freshened 240 brake pad's worth of travel is roughly* equivalent to 4 feet of 3/16 brake line. With two pots per line on the fronts, a just-in-time pad replacement on the left-front wheel might send gunk from the calipers all the way home.

* roughly sez the pots are 1" dia, 1/2" travel, and the 3/16" line is 1/8" ID.

Art...this may also have happened to me: when I went to to the rear pads, the pads were so worn down that the brake failure light was on constantly. Maybe this was the black crud in the reservoir...?
 
Good man.
Bench bleed it &

<a href="http://media.photobucket.com/image/castrol lma/fjcamper/CastroLMA.jpg?o=0" target="_blank"><img src="http://i258.photobucket.com/albums/hh264/fjcamper/CastroLMA.jpg" border="0"></a>
 
Art...this may also have happened to me: when I went to to the rear pads, the pads were so worn down that the brake failure light was on constantly. Maybe this was the black crud in the reservoir...?

The shuttle trips the warning switch when the pressure is different in the circuits.
It has nothing to do with pad wear.

Read section 5 that I posted again.
 
Last edited:
Good man.
Bench bleed it &

<a href="http://media.photobucket.com/image/castrol lma/fjcamper/CastroLMA.jpg?o=0" target="_blank"><img src="http://i258.photobucket.com/albums/hh264/fjcamper/CastroLMA.jpg" border="0"></a>

That's what I got. I was going to use ATE Super Blue but if I need to do brakes again in a pinch I don't want to have to worry about where to get it...it is not available in my area. The Castrol is available at almost all auto part stores here.
 
The Castrol GT LMA brake fluid can be tricky to find quickly too.

It is bettr than the regualar Castrol DOT 4 brake fluid.

The "LMA" designation, means it absorbs less moisture, as quickly, as regular DOT 4 fluid.
 
Here in my area AutoZone usually has a couple bottles on the castrol LMA.

I used the ATE Super Blue and bought a few extra cans for later on. Brake fluid does have a shelf life I'm told. But seems no one really knows how long that is for unopened bottles/cans.

Once you open a bottle, use it all . Ya really should not use a bottle that has been opened and put back on the helf for much of a period of time.

Never add to brake system with a bottle that has been sitting around a long time after it was opened. You'll be adding moisture to the system if you do...

Brake fluid is hygroscopic. it'll absorb moisture from the atmosphere.
 
Here in my area AutoZone usually has a couple bottles on the castrol LMA.
Once you open a bottle, use it all . Ya really should not use a bottle that has been opened and put back on the helf for much of a period of time.

Good advice for topping off as needed.
Can't wait to start this and get that gritty feeling on my finder-tips from the brake fluid once my gloves tear!!
 
where's the warning switch?
The earlier 240s (pre-ABS I think) had a switch on the distribution block, located above the left front (drivers side) control arm. They eventually leak, fluid level drops, pedal gets low when the level gets low enough. Top up the fluid, brakes feel normal again. This can go on for a long time if you don't mind buying brake fluid.
 
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