Leaking master cylinder...

[colkking]

Help!
I have now changed the master cylinder three times on my MUTT (M151). The first (came with the vehicle) had a crack through the threads on the bottom fitting. The second one had a leaking bottom fitting as well. The third is leaking from the top side...I am confused about this one.
Any ideas on what the cause is for this latest setback, and any ideas on how to fix it?
If not, any good sources for a GOOD master cylinder?

It is the last 5% getting this MUTT together that is driving me crazy...

Thanks in advance,
kk

[m3a1]

You should have a copper washer at the bottom fitting. Sounds to me like you don't if you have been having problem after problem. You will see that a new copper washer has some raised circumferential rings on it, like a bull's eye. When the fitting is tightened down, these rings squash down and create a very adequate seal. Reusing the old. flattened copper washer might make a seal...and it might not. Any reputable auto parts store will have what you need, though you may find you have to buy the assortment pack, which seems to be the way things are going nowadays..

Leaking at the bottom doesn't explain leaking at the top, though I will say that it is very difficult to add fluid without overfilling this style of master cylinder. That over-filled fluid will disappear from view immediately, and it will find it's way downward, over the outside of the master cylinder and other things under there where you can't see it....and it may take its good old time doing so, but eventually you will find it again, dripping on the floor....leading you to believe it's leaking at the top. The brake fluid shouldn't be filled that high in any case. So, are you SURE that it's leaking at the top?

By the way, DOT 5, being silicone, may make your floor and pedals slippery which isn't a good thing.... but on the plus side, it's a darned good metal preservative!

Cheers,
TJ

[colkking]

Thanks, TJ!
I learned my lesson on the copper washers on the second iteration--I did end up buying the 'assortment pack' at NAPA, and made sure that new washers were in place (I think this was the issue with the second master cylinder). I even put the brake fitting tee and banjo bolt on the master cylinder before I installed it this time.
Definitely looks like it is coming from the top of the cylinder--I will wipe it down tonight and see where it is coming from in the morning.
Yah, this one had my stumped.

On the plus side, the engine did start up this evening with no problems.

Thanks,
kk

[D Pizzoferrato]

I once boiled the silicone fluid out of my master cylinder. I discovered that the stop light switch was oriented too close to the back of the dash, putting tension on the terminals. I determined that the potting material was compromised in the switch, allowing the fluid to act as a conductor to ground, boiling the contents of the reservoir. A new switch and a better orientation took care of the problem.

[rickf]

Spray the whole cylinder down with brake cleaner and then see if you are still getting a leak after it dries. Brake cleaner dries very quickly and will not harm the paint if it is good paint.

[colkking]

Well, figured out the source of the leak!
The cylinder was leaking from the top...the disc on the top of the cylinder body was loose, and discharged brake fluid as soon as the system was pressurized.
On to master cylinder number four!!!
...and I will use NEW copper washers yet again....

I now have removal of the master cylinder down to 16 minutes....so at least I got that going for me!

[rickf]

There is no pressure in the main body of the master cylinder? It is just a reservoir for fluid. The screw on lid has a vent in it. Unless there is air in the system there should be almost no fluid motion in the main body of the master cylinder.

[colkking]

Hi rickf!
Not the screw on cap, the expansion cap on the top of the cylinder was not peened down and therefore was loose. When the pedal was pushed, instead of the hydraulic force being transferred by the fluid to the wheel cylinders, the fluid found relief by escaping through the unpeened and loose expansion cap. The QC guy at Wagner was asleep when this one went down the line...I should buy a lottery ticket...

[m3a1]

REALLY?! Good grief. Perhaps you should buy a lottery ticket...just don't leave the house to do it. You're a marked man!

[colkking]

can't make this stuff up! --smh--
can only laugh and learn

[fergrn37]

Expansion cap? Please explain or pic...

[rickf]

I am going to have to find an old MC around here. I have never heard of an expansion cap. Never remember seeing anything like that. The only openings I ever remember seeing were the rod for the pedal, the line connection and the cap for filling, that's it. The rest of it was a casting.

[Surveyor]

I'm guessing that goldish colored circle on top...

Sometimes things go great, other times I think the cast of Heehaw is standing behind me.
If it weren't for bad luck, I'd have no luck at all
Gloom, despair, and agony on meeeeeeeeee

[rickf]

Now that I see that I am thinking of a conversation I had with someone a while ago about the master cylinder for a 116 trailer and thinking it may be the same as the 151 since it bolts in the same. This may be the fill for the 116 and the other one would have a blank cap on it. Don't quote me on that because I am not sure. I need to go out and take my 116 trailer apart that has the bad cylinder on it.

[colkking]

The cap is there for the manufacturing process of the casting...in order for the internal cavity to be able to be hollow, this type of casting done as a sand casting. The sand core is placed inside the casting mold, and the liquid metal is poured in around the sand core. Once the mold is filled with the liquid metal with the sand core, the casting is removed from the mold, the sand is removed (usually by a shaking machine), and then the orifice to remove the sand is covered by a cap like this. In order to secure the cap, it is placed in as a dome shape, then peened down to expand it and secure the orifice....in my case, Carl on the line didn't peen the cap. Lucky me.

How do I know this? I used to work in an aluminum foundry