Master Cylinder leak

[shivelresk]

From where the MC connects to the bottom of the brake power booster there is a raised portion, then a lower section, then back to raised. In the first raised area, closest to the booster, there is a small hole at the top (see diagram below). Brake fluid is leaking out of this hole, best I can tell that is, and running down the power booster. Is there actually supposed to be something connected there? Can anyone please tell me what it means when this is happening and what, if anything I can do to fix it. Any help would be appreciated. erik.


...................................|
____MC top_______ |
________________ |
____side view_____|
________________ |....Power
____MC bottom____|....Booster
|_______________ |
..........|____|.....|)<--|----------------hole
..........|___|........|__ |
..........|__ |.........|__ |
..........|__|..........|_ _|
.................................|
..................................|
...................................|

sorry can't get the diagram to come out any clearer.

 

[Eric]

Hi Erik and welcome to the CAC!

I thought I had the exact same leak as you are describing. I went so far as replacing the MC. When I replaced the MC I used the old cap because it looked more original than the shiny new one. Well low and behold the same leak showed up on the new MC. Turns out the true culprit was the cap. I had a bud jam on the pedal and, sure enough, fluid oozed out of the passenger side of the cap. However, I never even suspected the cap since I was feeling the fluid running down from what I thought was the spot you describe then dripping from the low spot. I put the new cap on and no more leak. This may not be your issue but it's worth checking.

Good Luck!

 

[shivelresk]

I appreciate the info. however this is not the cap as a I have looked the entire thing over and it is only wet from that bottom area. I actually had leveled off the fluid last evening, and I came home to find a good bit of it on the garage floor. I haven't even driven it or pushed the brake pedal since re-filling the fluid. Also I took the cap off and the cone closest to the firewall on the cap's inner lining was bubbled out, and the fluid was lower again.

erik.

 

[Chick]

Time to replace the master cylinder, it's not uncommon to leak in the back in that spot, just replace it.

 

[shivelresk]

Could it not possibly be a seal or something that pertains to the piston, at the point of connection with the power booster. Something which could be fixed by purchasing a rebuild kit as opposed to buying a totally new master cyliner.

 

[Jack]

If you have a parts house handy ... don't sweat rebuilding it ... bought complete rebuilt M/C for my 71 at either advance or autozone ... less than $25 outright ... came with a new lid & gasket. Dunno if M/C's are date-coded or not ... I kept my old one.
JACK:gap

 

[cruzer99]

I am not sure about your MC but most have a "tattle tail hole or relief" that will show fluid running down the booster when the seal is bad. I agree with Jack, replace it.......

 

[smurfvet]

The main master cylinder seal is leaking. The hole is a "weep hole" that allows brake fluid to drip out of the area if the seal heads downtown. Change the MC. You can try to rebuild it, but you will probably need to remove the old "mud" left behind from the brake fluid deteriorating and the seal deteriorating. You will also have to probably hone the inside of the MC where the seal goes. If you've never done it before it's a lot easier to just get a rebuilt.

When you put the cylinder in, notice if there is any of the dirty fluid in the lines going from the MC to the brake distribution/proportioning block. The block is under the MC mounted on the left frame rail almost to the lower frame. If there is "mud" in there, flush all the lines right back to the calipers.

Sounds tedious, and it is. The problem is if that mud gets to stay in the calipers, then the seals in them start leaking. No brakes is a bad thing.

 

[cruzer99]

One last and painfull thing.......Make sure you bleed the brakes in the sequence/steps in the manual. I'm assuming it is a dual brake system and they not fun to bleed. You will know you got it right if the dash brake light is off......I know I shouldn't ASSUME.....Good luck