1983 300SD Master Failure
 

I live an exciting life. The master failed on my 1983 300SD. I was able to get her stopped by some mad pumping as I approached a busy intersection. It went to the floor with no warning. The problem is that the master was rebuilt last year! So I ordered a new one last week and it happened again. There is no leakage from any line, caliper or master seals . Everything is dry. When bleeding the new master on the bench I noticed the far port that connects to the distribution block for the rear wheels didnt pump fluid, it just came out up the tube to the fluid level. The other two ports that go to the front wheels pumped up fine. Could this new master be bad also? Should there be a O-ring around the back going into the booster? Mine didnt have one on it when I removed it. Maybe the booster failed...not that it has anything to do with the master itself but as an assisting device. The brakes worked today when I moved the car by coasting it in. :(

Did not pump fluid... but you installed it anyway ?

Yes, a new one... probably rebuilt .. can be bad upon arrival...

How did you bleed your rear brakes ?

Yes, you need a good vacuum seal between the MC and booster. The flat O-rings that should come in the box are often too thin. I recall using a round O-ring once from my kit. In older U.S. cars, there is no such seal. The booster has an inner bellows that seals the vacuum, but those boosters are thicker. Even a 99 Breeze booster I have is like that. The booster on my minivans is thin like the 300D, so I expect it similarly requires an MC seal.

I have no idea why you got no fluid from the rear port when bench bleeding the MC. Insure the fluid level was high enough to flow into the rear brake reservoir (usually the fwd one). Insure the port was open (no left-over dust seal). I think MC's have an internal check valve that allows it to fill from the reservoir and eject only to the port. Perhaps the check valve is stuck open and just needs a little persuasion (rapid pumping,...).

Yes, I know it was a stupid thing to do but I trusted that a new MC will work better than a rebuilt. I re-installed the old unit that I rebuilt a year ago. I tested it and it pumps great through all three ports. Leads me to believe that it is not the problem. The brakes work great cold. After a decent run they failed with both MCs.

I suggest you start back at the basics... flush your whole system and put in brake fluid from a newly opened can.

If you have moisture.... once hot... can turn to steam....and cause very strange symptoms...... since you describe this as after some use.. that may be the case...

When I took off the new one I put on last week, I heard the vacuum break. There is a thin flat seal there. I re-installed the old one I took off as it tested good on the bench. Better than the new MC as this one I know works.

Both reservoirs were filled up completely on the bench and in the car. Also pumped it up rapidly and held the pressure for a minute or so, no leaks.

That is an interesting thought. Ill pressure bleed the complete system and see what happens. Maybe a little water got in somewhere.

It can also be possible you are experience a stuck caliper, if a caliper sticks it will also cause the peddle to hit the floor...

What are the physics of that situation ? I can not visualize it myself....

Only thing I can think of would be for the stuck caliper to boil the fluid resulting in vapor at the pedal...

I have no idea truthfully, I know the flex lines can act as one way valves as they get older. Thus the fluid cannot return to the tank. So I imagine if a piston gets stuck in a caliper the same can happen. I had it happen to me when I got my 78 running after sitting....had to drive back home with the e brake...as soon as I bleed the system all was fine..

I had that happen to me also... the hose simply had a flap come loose inside... it locked up that caliper .....
but I do not understand how that could cause the pedal to go to the floor....

Did your warning indication lamp come on since the one circuit (rear) was not working? When you rebuilt your MC are you certain you replaced all the rubbers for both pistons, I know people who don't pull the second piston. Was the cylinder honed and cleaned properly? I glad there was not an accident and it seems you are not badly shaken these things can be scary.

I wonder where the "new" master cylinder came from. The pedal only goes to the floor if there is compression, a leak or if the pedal is disconnected and moves independently of the MC.

Cheap new parts don't necessarily work. Cheap remans don't necessarily work. One time I installed an SD master without bench bleeding. I expected pressure bleeding to take care of it. I ended up pulling it off and bench bleeding then power bleeding the entire system.

The pedal didn't feel completely right until I set all wheels on the ground and went for a drive. Brokers still work perfectly. The MC was new from this site back when Roy still took care of things.

No indicators came on. Took it for a spin this morning and the brakes are fine now. I placed the MC I rebuilt back in, bled the entire system. Now the car stops as it should even after a long drive and aggressive stopping.

When I did the brake job, it was wet and humid out here so there may have been some water that got in and caused this malfunction.