Leaking brake master cylinder
Thread Starter
Instructor
Joined: Aug 2015
Posts: 147
Likes: 9
Leaking brake master cylinder
So the other day I did my brakes (hoses, pads, rotors, and full system flush) and now the brake master cylinder / booster area is leaking! 
When I flushed the system and replaced the fluid I replaced it with this DOT 3 & 4 fluid from Valvoline. http://www.autozone.com/brake-and-po...?checkfit=true
I didn't do anything to the master cylinder / booster area except open the caps and fill the reservoirs. Now I've got a leak between the master and booster. The leak is on the rear brake part of the system because that reservoir was about half full when I got home last night. I found drips on the bottom of the booster and traced them back up to the joint between the two parts.
So, did I do something wrong? We bled the system with the 2 person method and vacuum methods and got all the air out. Could pushing the pedal so many times during the bleeding process have done something? Is it possible that the new 3 & 4 fluid is thinner than the brake fluid that was in there and that's what's causing the leak?
The other question is can the problem be fixed without having to buy a new master cylinder, booster, or both? Is there a gasket or something between the two that has gone bad? I'm not sure what's in there so I'm not sure what can/should be replaced. Any thoughts would be appreciated.
When I flushed the system and replaced the fluid I replaced it with this DOT 3 & 4 fluid from Valvoline. http://www.autozone.com/brake-and-po...?checkfit=true
I didn't do anything to the master cylinder / booster area except open the caps and fill the reservoirs. Now I've got a leak between the master and booster. The leak is on the rear brake part of the system because that reservoir was about half full when I got home last night. I found drips on the bottom of the booster and traced them back up to the joint between the two parts.
So, did I do something wrong? We bled the system with the 2 person method and vacuum methods and got all the air out. Could pushing the pedal so many times during the bleeding process have done something? Is it possible that the new 3 & 4 fluid is thinner than the brake fluid that was in there and that's what's causing the leak?
The other question is can the problem be fixed without having to buy a new master cylinder, booster, or both? Is there a gasket or something between the two that has gone bad? I'm not sure what's in there so I'm not sure what can/should be replaced. Any thoughts would be appreciated.
Last edited by DarkObsession; May 17, 2016 at 01:34 PM.
Race Director
Joined: Oct 2008
Posts: 17,982
Likes: 1,074
From: Lansing MI
So the other day I did my brakes (hoses, pads, rotors, and full system flush) and now the brake master cylinder / booster area is leaking!
When I flushed the system and replaced the fluid I replaced it with this DOT 3 & 4 fluid from Valvoline. http://www.autozone.com/brake-and-po...?checkfit=true
I didn't do anything to the master cylinder / booster area except open the caps and fill the reservoirs. Now I've got a leak between the master and booster. The leak is on the rear brake part of the system because that reservoir was about half full when I got home last night. I found drips on the bottom of the booster and traced them back up to the joint between the two parts.
So, did I do something wrong? We bled the system with the 2 person method and vacuum methods and got all the air out. Could pushing the pedal so many times during the bleeding process have done something? Is it possible that the new 3 & 4 fluid is thinner than the brake fluid that was in there and that's what's causing the leak?
The other question is can the problem be fixed without having to buy a new master cylinder, booster, or both? Is there a gasket or something between the two that has gone bad? I'm not sure what's in there so I'm not sure what can/should be replaced. Any thoughts would be appreciated.
When I flushed the system and replaced the fluid I replaced it with this DOT 3 & 4 fluid from Valvoline. http://www.autozone.com/brake-and-po...?checkfit=true
I didn't do anything to the master cylinder / booster area except open the caps and fill the reservoirs. Now I've got a leak between the master and booster. The leak is on the rear brake part of the system because that reservoir was about half full when I got home last night. I found drips on the bottom of the booster and traced them back up to the joint between the two parts.
So, did I do something wrong? We bled the system with the 2 person method and vacuum methods and got all the air out. Could pushing the pedal so many times during the bleeding process have done something? Is it possible that the new 3 & 4 fluid is thinner than the brake fluid that was in there and that's what's causing the leak?
The other question is can the problem be fixed without having to buy a new master cylinder, booster, or both? Is there a gasket or something between the two that has gone bad? I'm not sure what's in there so I'm not sure what can/should be replaced. Any thoughts would be appreciated.
Your going to have to replace the master cylinder. Its possible that someone offers a rebuild kit but I expect that's going to be much more hassle than its worth. The booster should be fine; I would at least try it.
Last edited by PatternDayTrader; May 17, 2016 at 02:53 PM.
Oil Producer
Joined: Apr 2014
Posts: 9,377
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Hi Darkob,
In my case, on my 85, i thought i had the same situation. and it had been happening for years before i owned the car because the paint on the frame beneath the assembly is all gone.
in my case, it was actually BOTH orings on the master cylinder reservoir. the 85 has two reservoirs, both were leaking.
get a flashlight, and get in there close and inspect the rubber rings at the base of the reservoirs. they might be seeping. if that is the case, they are cheap, 7$ each, i bought them off zips.
getting them back on was really hard, until i opened my shop manual and discovered the trick was to use a dollar store metal turkey baster hacksawed off at the end. thread is here if you need it. cheers and hope you get it fixed. https://www.corvetteforum.com/forums...reservoir.html
In my case, on my 85, i thought i had the same situation. and it had been happening for years before i owned the car because the paint on the frame beneath the assembly is all gone.
in my case, it was actually BOTH orings on the master cylinder reservoir. the 85 has two reservoirs, both were leaking.
get a flashlight, and get in there close and inspect the rubber rings at the base of the reservoirs. they might be seeping. if that is the case, they are cheap, 7$ each, i bought them off zips.
getting them back on was really hard, until i opened my shop manual and discovered the trick was to use a dollar store metal turkey baster hacksawed off at the end. thread is here if you need it. cheers and hope you get it fixed. https://www.corvetteforum.com/forums...reservoir.html
So the other day I did my brakes (hoses, pads, rotors, and full system flush) and now the brake master cylinder / booster area is leaking!
When I flushed the system and replaced the fluid I replaced it with this DOT 3 & 4 fluid from Valvoline. http://www.autozone.com/brake-and-po...?checkfit=true
I didn't do anything to the master cylinder / booster area except open the caps and fill the reservoirs. Now I've got a leak between the master and booster. The leak is on the rear brake part of the system because that reservoir was about half full when I got home last night. I found drips on the bottom of the booster and traced them back up to the joint between the two parts.
So, did I do something wrong? We bled the system with the 2 person method and vacuum methods and got all the air out. Could pushing the pedal so many times during the bleeding process have done something? Is it possible that the new 3 & 4 fluid is thinner than the brake fluid that was in there and that's what's causing the leak?
The other question is can the problem be fixed without having to buy a new master cylinder, booster, or both? Is there a gasket or something between the two that has gone bad? I'm not sure what's in there so I'm not sure what can/should be replaced. Any thoughts would be appreciated.
When I flushed the system and replaced the fluid I replaced it with this DOT 3 & 4 fluid from Valvoline. http://www.autozone.com/brake-and-po...?checkfit=true
I didn't do anything to the master cylinder / booster area except open the caps and fill the reservoirs. Now I've got a leak between the master and booster. The leak is on the rear brake part of the system because that reservoir was about half full when I got home last night. I found drips on the bottom of the booster and traced them back up to the joint between the two parts.
So, did I do something wrong? We bled the system with the 2 person method and vacuum methods and got all the air out. Could pushing the pedal so many times during the bleeding process have done something? Is it possible that the new 3 & 4 fluid is thinner than the brake fluid that was in there and that's what's causing the leak?
The other question is can the problem be fixed without having to buy a new master cylinder, booster, or both? Is there a gasket or something between the two that has gone bad? I'm not sure what's in there so I'm not sure what can/should be replaced. Any thoughts would be appreciated.
Thread Starter
Instructor
Joined: Aug 2015
Posts: 147
Likes: 9
Thanks for the replies. After looking at the system again I'm pretty sure the failed part is at the rear of the master cylinder. I looked and felt around on that part and didn't see/feel any fluid. Seems like it's leaking from the connection to the booster so my guess is that it's the master cylinder's rear seal. I ordered the part tonight so hopefully it'll be here soon. I'll report back when I know more.
Drifting
Joined: Sep 2014
Posts: 1,579
Likes: 59
From: Trieste Italy
So the other day I did my brakes (hoses, pads, rotors, and full system flush) and now the brake master cylinder / booster area is leaking!
When I flushed the system and replaced the fluid I replaced it with this DOT 3 & 4 fluid from Valvoline. http://www.autozone.com/brake-and-po...?checkfit=true
I didn't do anything to the master cylinder / booster area except open the caps and fill the reservoirs. Now I've got a leak between the master and booster. The leak is on the rear brake part of the system because that reservoir was about half full when I got home last night. I found drips on the bottom of the booster and traced them back up to the joint between the two parts.
So, did I do something wrong? We bled the system with the 2 person method and vacuum methods and got all the air out. Could pushing the pedal so many times during the bleeding process have done something? Is it possible that the new 3 & 4 fluid is thinner than the brake fluid that was in there and that's what's causing the leak?
The other question is can the problem be fixed without having to buy a new master cylinder, booster, or both? Is there a gasket or something between the two that has gone bad? I'm not sure what's in there so I'm not sure what can/should be replaced. Any thoughts would be appreciated.
When I flushed the system and replaced the fluid I replaced it with this DOT 3 & 4 fluid from Valvoline. http://www.autozone.com/brake-and-po...?checkfit=true
I didn't do anything to the master cylinder / booster area except open the caps and fill the reservoirs. Now I've got a leak between the master and booster. The leak is on the rear brake part of the system because that reservoir was about half full when I got home last night. I found drips on the bottom of the booster and traced them back up to the joint between the two parts.
So, did I do something wrong? We bled the system with the 2 person method and vacuum methods and got all the air out. Could pushing the pedal so many times during the bleeding process have done something? Is it possible that the new 3 & 4 fluid is thinner than the brake fluid that was in there and that's what's causing the leak?
The other question is can the problem be fixed without having to buy a new master cylinder, booster, or both? Is there a gasket or something between the two that has gone bad? I'm not sure what's in there so I'm not sure what can/should be replaced. Any thoughts would be appreciated.
