1983 240D - did I trash my master cyl while pump-bleeding?
 

I replaced both rear calipers and now I'm having trouble bleeding. At first, I had a bad bleeder nipple on one of the ATE rebuilt cylinders. It had a burr on where it seats. When I discovered this, I was getting good pedal, but could make the bleeder not leak- so got a replacement bleeder.

I had my son come help and we did the "pump-hold-bleed" method.

Next day (today), I put on the new bleeder and try to bleed it out. No pedal- and nothing comes out of the bleeders!

Also, I put in DOT4 - should I only use DOT3?

Somehow, I seem to have messed up the master cylinder seals so now it doesn't work at all? I get no pedal.

I'm assuming this is the case- but I have been bleeding brakes this way for 40 years with no problems...

Are you sure there's not just air in the system? If you had a leaky bleeder, it's possible you're air-locked. If you got air all the way back up to the master cylinder, it can be a real PITA to bleed the air out (especially if you have ABS). Look up how to do a "bench bleed" on the master cylinder if you suspect it is airlocked.

You should be able to use DOT4 in a DOT3 system. The Pump-Hold-Bleed method of purging air has been a standard practice since the introduction of hydraulic brakes. Highly doubtful you damaged anything in the master cylinder. If it did let go, it was just that the rubber was deteriorated enough that the repeated pumping to bleed the system finally wore it down to where it now leaks. My money would be on an airlock.

When you release all the pressure on the hydraulic system it allows for the pumping action to push the piston too deeply into the cylinder.

Kent Bergsma suggest to not do the pump hold thing for old MB cars for this reason- the mater cylinder goes to a place that it has never traveled before and the seals can get messed up.

This car sat undriven in a barn for 15 years, so it is probably good to replace all brake components on principle anyway.

Regarding "air lock" I don't know how this could have happened- since the brakes were working pretty well after caliper install and initial bleeding. Then I discovered the bad bleeder, took it off (and left the hole for the bleeder open overnight). Maybe leaving it open allowed a bunch of air to get back into the system.

I also tried using a vacuum bleeding approach- first a mitivac, then a harbor freight vacuum bleeder. The latter was getting good suction, but didn't pull much fluid through. Kinda baffled.

Yes, pushing the pedal all the way down past normal travel can damage the seals on used master cylinders. Not sure about new master cylinders but definitely on used.

Good luck!!!

'70 220D:
Tried the old school method of "pump up and hold it". Did not like the feel of the pedal.
I bench bled the MC prior to installation. Purchased a high quality vacuum method for
brake bleeding (hand held) and what a huge difference. Had them bled in no time. I replaced all (4) calipers, MC, and booster. Stops on a dime, and gives you change!
Might want to check your hoses as well. Mine were a mess internally. Found replacements at NAPA - with the Semperit brand on them.

The pump and hold and open the bleeder is perfectly good process..

THE PROBLEM

happens IF the person on the ground in charge of opening AND closing the bleeder does not understand that they are to

ONLY open it a little... and close it immediately...

PATIENCE is what is needed.... that high pressure push from above using NEW proper fluid does a great job...

but if you keep the nipple OPEN TOO LONG... then you can allow the master cylinder to travel too far... as has been described above...

It is pretty much just open and close as fast as you can do that...

People who are in a hurry or do not understand the parts involved.. should not be doing brake work... no short cuts are allowed on brake work....

Although the description above will stand on the physics alone... for the record.. I used to work at a Spring and Brake shop....

Yeah- I was leaving the bleeder open too long. I've never heard the admonition of cracking it briefly before. Makes sense, tho.

So- the question is- do I go ahead and do the master cyl - esp since it is old and sat so long unused? I've done them before and they are not too much work.

Yes, rebuild master cylinder. Brakes are an important safety item.

And replace the hoses...soak the connections early on.

If I was to ever replace a master cylinder again, I would just spend the money and buy a pressure bleeder.

I certainly suggest a new master cylinder... do not cost much relative to their importance... and in an easy place to deal with..
IF it says ' bench bleed' in the instructions....... I suggest you follow that instruction...
In the archives I think it says a kit can be had for that bench bleeding from NAPA... it is just a few lines and which fit the holes.. you just put the output so it feeds back to the reservoir and pump the piston until no air is in the system... very easy and many a person thought they could do that ON the car.... and had to take it back off and follow the directions...

I do not own a pressure bleeder.. but if one does not have a helper available .... or a good one... they make it where YOU are in charge of that squirt.... and I guess it means the squirt does not need to be limited in the way it does depending on the movement of the master cylinder piston... they may waste some fluid due to the fact that any left over in the power supply is contaminated , having been exposed under pressure to AIR.... but the trade off seems pretty reasonable....

Occams' razor!
 

The problem is you allowed the rear compartment of the brake fluid reservoir to be pumped/run dry!

On these MBs the reservoir is separated into two distinct reservoirs, one for the front brake circuit and another for the rear brake circuit. It is quite dificult to see if you don't know exactly what you're looking for. It helps very much if you use a flashlight to illuminate the translucent plastic of the reservoir.

You need to slowly and carefully fill the front section until it overflows into the rear section and then keep that up as you cycle new fluid into the rear brakes.

"Among competing hypotheses, the one with the fewest assumptions should be selected."

EVEN IF there is fluid in both front and rear reservoirs the MC can be messed up by allowing the piston to travel too far...by allowing too much brake fluid to escape during any one ' open nipple ' squirt.... typically the piston moves into a section of the bore which has corrosion ... and messes up the seals.

The MC was pretty much in need of replacement even before you tried to use it. While sitting that long, the entire bore is now considered unused.

With the bleeder screw removed, the system gravity bled and most / all of the fluid could have fallen out. The only way this won't happen is if the MC cap is sealed and won't let air in, at this point fluid will stay in the system. ( Think drinking straw filled with liquid with the upper end crimped off and held vertically. )

The way I see it is if the MC can't tolerate a full stroke bleeding event, it needs to be replaced. Think about what happens in a dual circuit MC system if one circuit develops a leak, pedal travel will go full stroke to allow the other circuit to operate. If the MC can't tolerate this, you will have a total loss of braking. ( Google how a dual ckt MC operates, there has got to be a vid somewhere )

Well we'll have to wait and see what the actual outcome is I suppose; but, the OP originally reported no problem getting sufficient pedal and pressure prior to the problem with his not being able to seal the bleeder to be leak free. Then he mentions leaving the bleeder out and the system open for a period of time. Doing that will allow all the fluid to drain out of that caliper's vacant bleeder screw opening. Then later the OP details he applied vacuum to the bleeder and could not draw any fluid, if there was fluid in the rear reservoir he would have drawn it towards the caliper.

All those things considered I'll stick with the drained rear reservoir compartment until more information pointing to some actual MC malfunction is revealed.

The good thing is it will be quite easy and inexpensive for my theory to be confirmed or refuted whereas fooling with the MC will require much more effort and expense to verify the theories around that.

I agree the old unit needed replacement due to lack of use and age..

but your description of pedal travel going to full stroke to allow the other circuit to operate is not correct... both front and rear are operating independently and at the same time...

There can be differences built into the system due to the amount of fluid required for regular brakes on the rear and discs on the front.. but those are adjusted by things like bore size...so the same pedal travel stroke produces those needed differences....

https://www.google.com/search?q=master+cylinder+dual+circuit&biw=939&bih=602&tbm=isch&imgil=A-S_pFUtkEZ4KM%253A%253B8nCRTg4G0hK-KM%253Bhttp%25253A%25252F%25252Fwww.tegger.com%25252Fhondafaq%25252Fmastercylinderreplace%25252Fhoww orks.html&source=iu&pf=m&fir=A-S_pFUtkEZ4KM%253A%252C8nCRTg4G0hK-KM%252C_&usg=__qhrgALcZGJRLw088q7PD1cSSc0I%3D&dpr=1.09&ved=0ahUKEwjv7IWG2MfQAhXnilQKHS_XCHYQyjcIbw&e i=1Ck6WO-yCeeV0gKvrqOwBw#imgrc=A-S_pFUtkEZ4KM%3A

You have miss quoted me and have absolutely no idea how a single bore dual circuit brake master cylinder operates.

Try reading what I posted again without cutting relevant information out of the middle.

I did not misquote you because I did not change anything you said... I just left out the part which did not pertain to what I was pointing out was not true.

Did YOU go and look at the pics in the google article ?

This statement "pedal travel will go full stroke to allow the other circuit to operate."
is not true... both the front and rear operate upon the same stroke. One is not having to travel full stroke... ' to allow ' the other to operate.

I agree with this. I prefer the simplicity of the old two person brake bleeding process. I don't see a healthy mc being damaged by this process.

I've highlighted the portion of my quote you deleted and colored red the portion vital to the point I was making.

97 SL320 One circuit developing a leak has nothing to do with the functioning of the other section of the circuit. It does not have to do anything to ' react ' to the leak in the other side... it is oblivious to that leak.. it just functions according to the physics incorporated in how it was constructed.... a bore with a sealing piston pushing brake fluid...

Losing fluid in one circuit doesn't force the other to go to full stroke to allow braking. The pushrods are a solid connection and will operate regardless of the fluid level.

What WILL happen is a dramatic change in FEEL of the brake pedal, making it far mushier (is that a word?) since you're now only compressing half of the working fluid. This does allow you to push harder on the pedal, but it isn't the result of pushing one of the circuits through full stroke to operate the other.

You should buy a pressure bleeder. I have one and it has allowed me to bleed my brakes all by myself. Also, you can do a brake fluid flush to get rid of all the old brake fluid. FYI, you should also change out all 4 brake lines because they do break down internally from old brake fluid, it's cheap insurance from your brakes locking up and not releasing.

I agree... which particularly pressure bleeder did you buy ? which you are happy with.. OR would you get a different model had you known about the advantages in the designs ?

I got the Motive one from Amazon with the European adapter. I actually fill the master cylinder to the very top with brake fluid instead of filling the Motive pressure bleeder, there is no cleanup after you are done! Also, you can purchase other master cylinder caps for your pressure bleeder to work on other vehicle makes.
I should have bought the Motive pressure bleeder 20 yrs ago when I first started working on cars!!!!

The push rod is only attached to the left piston, a column of fluid operates the right piston as there isn't a rigid connection between the two. If there was a rigid connection, the braking system half that had the least free travel would cause the piston to stop moving before the other had a chance to build sufficient pressure.

The pic referenced in LMs post shows a spring between the two pistons, this is merely one of two return springs.

When the column of fluid is lost between the two pistons due to a leak ( left brake line ) , the left piston must travel until it contacts the safety stop on the right piston. At this point the right piston is moved to generate hydraulic pressure. The pic referenced by LM makes note of this " This part begins to compress."

If the leak occurs on the right piston, this right piston must bottom out in the bore before the left piston can generate any pressure.

In either case, a leak will allow one of the pistons to travel full stroke.

You looked at the wrong picture.

http://www.gomog.com/allmorgan/MC1.jpg

I also fill the master cylinder to the top but did not spending $ buying a Motive. I took a spare cap, drilled a hole and put a tire valve (found strewn all over the ground at tire shops) in the hole. Apply compressed air with a hand bicycle pump and that's my pressure bleeder setup. No cleanup other than wiping down the tire valve and cap when done.

It's interesting how LM says I'm wrong and provides a web site that proves what I'm saying is correct. Still want to go another round?

MASTERCYLINDER

For the complete story with pics that matter, use the link provided. ( The MC in the pic does not have the 2 return springs, apparently they have been omitted for clarity. )

97 SL320
You are describing a sequential operation when our brake master cylinder front and rear are acting simultaneously.


http://www.gomog.com/allmorgan/MC1.jpg

Dual-Circuit master cylinders don't operate "front and rear" of the car, they operate opposing corners (front left, rear right for example). The push-rod pushes on both circuits at the same time. There is a bit of "slop" in the fit between the first and second piston and the spring does take up the slop to an extent, but it isn't full-stroke. Only enough to allow some equalization between the first and second piston. If the link works, you can see a better cutaway:

You can't make that as a blanket statement.
Subaru splits their units diagonally...
but many cars are split front/rear .

Diagonal split is the norm rather than the exception. The whole idea being that if you lose one circuit of the braking system, you can still safely stop without losing control. Having only the front or only the rear working can very easily cause a spinout or rollover.

The pic you reference is from the link I posted, and it shows operation when the "front" braking circuit has a leak. In this case, piston 1 travels much farther then normal until it contacts piston 2. This would be considered full stroke of piston 1.

A leak in any circuit = full stroke of the remaining good circuit The MC better be in good enough condition to tolerate this, if the MC can't tolerate full stroke bleeding, it needs replaced.

I agree that if the unit has corrosion which messes up the seals if the movement is allowed to go too far by the person in charge of the bleeder nipple at the wheel it should be replaced..

Have you looked at the MC picture in the FSM ?
The use of the word ' much ' and ' full stroke' etc... are a problem.

Not strictly true...

A leak in the Secondary circuit (end of the MC) would not cause a "full stroke" of the Primary circuit (in contact with the brake pedal pushrod). The Primary circuit will move only the amount needed to stop the vehicle, although the pedal will be "soft" due to the lack of hydraulic pressure on the Secondary circuit. Depending on the health of the brakes and design of the braking system, the stroke of the Primary will be more than "normal" but less than "full" unless a panic brake situation is encountered.

Conversely, a leak in the Primary circuit won't cause that piston to move "full stroke" to contact the Secondary piston either. The recess in the Secondary is enough to allow a difference in pressure between the 2 circuits, but not enough to allow the Primary to move it's full stroke before engaging the Secondary circuit.

The only way to have a full-stroke movement in either circuit is to have a leak in both and move the pedal to full-travel position. This is essentially what a "bench bleed" does since there isn't any hydraulic pressure on the system. You can move both pistons to their full maximum stroke to burp all the air out before installing in the vehicle.

You are correct though. If the MC is so worn out that a "full stroke" or "high pressure" plunge damages it, it certainly wouldn't survive a "panic brake" operation either. If bleeding the brakes killed the MC (bleeding is considered routine maintenance), it was on borrowed time to begin with.

This isn't entirely true, the split method depends on the the cars drive system.

A rear drive has always been setup with a front / rear split.* This works because the rear wheels have a decent amount of weight on them and will stop the car well enough on their own.

A diagonal doesn't work so well in a rear drive car because the steering is generally set up with little / no negative front scrub radius. If one front wheel has braking and the other does not, the car will tend to wind the steering wheel in the direction of the operating front brake.

* 80's Volvos used a 4 piston 2 circuit front caliper to make a triangular split using 1/2 of each front caliper and one rear. This gave near full front braking and the imbalance in the rear was manageable.


A front drive needs a diagonal split. With so much weight on the front wheels, losing front brakes and relying on rears will result in locked rear wheels and round the car goes. A front drive car can tolerate a diagonal split because it has more negative front scrub radius, this tends to turn the steering wheel in the direction opposite of the operating front brake countering some of the braking imbalance.

Scrub radius on a double a arm type suspension is found by drawing a vertical line through the tire tread then drawing a line through the upper / lower ball joints. The distance between where the two lines meet the road surface is the scrub radius.

If the LF wheel is pushed outwards from the center of the car, the scrub radius is now positive. Road irregularities / surface condition / braking imbalance will tend to steer the car to the left.

If the LF wheel is pushed inwards from the center of the car, the scrub radius is now negative. Road irregularities / surface condition / braking imbalance will tend to steer the car to the right. This result is helpful in reducing torque steer in a front drive car.

70s and 80s Fords and GM's were diagonal. In that era, they were almost all RWD. Their systems used a proportioning valve to allow different braking pressures to the front and rear wheels.

Gee, I've been doing this all wrong for the 40+ years I've been in and around the auto repair industry. It's no wonder that all the cars I've worked on and race cars I've built don't work properly. If you think the VW TDI recall is massive, just wait until I generate my own recall.

On the cars you reference, don't confuse both MC lines entering a frame mounted combination valve with an actual diagonal system. This combo valve contains a piston and switch to warn of a brake pressure imbalance and a proportioning valve that reduces rear braking pressure in proportion to front pressure, this isn't a regulator where outlet pressure is constant.

On the rear drive cars you reference, do they have one brake line running to the rear or two? Assuming a solid rear axle, is there one rubber brake hose running from the body or two? ( Short whip brake hoses from the brake caliper to the axle don't count on rear disc cars. )

The attached pic from this link looks like it was lifted from the factory VW manual.

| Repair Guides | Brake System | Basic Operating Principles | AutoZone.com

I will give that while a circuit might not go total full stroke, it will travel significantly more than normal when one circuit develops a leak / bleeder is opened to the point it might as well be considered full stroke.

Here is an animated MC with leaks in various circuits + dual circuit layout considerations.

1PORTER AND CHESTER INSTITUTE. 2 Brake Hydraulic Systems. - ppt download

brakes
 

Ironandsteel after reading all the posts of the ins, and outs, of the brake system .Did you manage to get your brakes to work after all ?