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 )