, I tried replacing the seal, but to no avail. The new cover thing seems like a good idea. 
Leaking master cylinder (1981).
Thread Starter
6th Gear
Joined: Jun 2007
Posts: 6
Likes: 0
Leaking master cylinder (1981).
The master cylinder on my 1981 is leaking from the cap. I replaced the cap, seal and the straps and it still leaks. I also cleaned the top of the master cylinder to make sure that the cap has a clean smooth surface to mate with, and it still leaks. Has anyone had this problem or have any solutions before I replace the master cylinder?
Thanks,
Anthony
Thanks,
Anthony
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Racer
Joined: Jul 2004
Posts: 286
Likes: 1
From: Mesa az
Cap and Seal Thought
If the seal is working like it's designed, it should not only be fluid-tight, but also air tight. The seal should have "balloon" sections that will expand into each of the resevoirs as the fluid level drops due to use and brake wear. The cover has holes that allows air in and out. The cap is NOT designed to be air tight, and in fact has to allow air in so that as the fluid drops the MC maintains atmospheric pressure. I've seen small indentions on the perimeter of the cap to allow air in, and also holes in the top of the two domes. The air goes into the top of the seal pushing it down into the resevoirs. The seal should keep air and contaminants out of the fluid, but must be flexible to allow for proper operation of the brakes.
Is the new seal you installed a solid piece of rubber that covers both resevoirs? Does it have the "balloon" sections that can move in to fill the resevoirs as the fluid level drops? Does the cap fit snuggly all the way around the top of the MC? Are your brakes working correctly? Does the fluid level seem to increase in the MC after extended use? If the system is somehow generating pressure in the MC it could "blow" the seal and force fluid out.
Just some thoughts.
Andy
Is the new seal you installed a solid piece of rubber that covers both resevoirs? Does it have the "balloon" sections that can move in to fill the resevoirs as the fluid level drops? Does the cap fit snuggly all the way around the top of the MC? Are your brakes working correctly? Does the fluid level seem to increase in the MC after extended use? If the system is somehow generating pressure in the MC it could "blow" the seal and force fluid out.
Just some thoughts.
Andy
Thread Starter
6th Gear
Joined: Jun 2007
Posts: 6
Likes: 0
The new seal is a solid one piece that covers both reservoirs and has the balloon sections. The braking system losses a little fluid over time do to a slight leak from one of the caliper piston seals ( I plan on rebuilding or replacing this caliper and if needed replace the MC at the same time). The cap fits snuggly all the way around the top of the MC. The leak occur regardless of trying three different seals, and two different caps/straps. The brakes work good however the pedal is a little low. I believe the reason for this is some air getting in the line from the leaking piston seal.
Safety Car
Joined: Jun 2004
Posts: 4,652
Likes: 80
From: Lakeland Florida
Instead of looking at the seals, look at the perimeter edge of the master cylinder with the top off. Run your finger around it and look for some type of abrasion, or a scratch mark.
If it is scratched then a leak could develop as this will not seal correctly and will not hold a vacuum.
kdf
If it is scratched then a leak could develop as this will not seal correctly and will not hold a vacuum.
kdf
Burning Brakes
Joined: Aug 2007
Posts: 1,084
Likes: 8
From: Illinois
I just bought a brand new MC and am having the same problem--small amount of fluid leaking from the top. Their suggestion was to check the edges for any rough areas and bend the two peices of metal that snap over the cover to hold it down.
Advanced
Joined: Dec 2007
Posts: 83
Likes: 2
From: Cramerton NC
Guys, I realize this is an old thread but Im having the exact same problem. Ive replaced the master with a new Delco unit, still leaks at the cap. Ive used 3 different caps with 3 different cap gaskets and 3 sets of bails and it still leaks at the back of the cap closest to the booster. The mating surfaces of the gaskets and the top of the master are not deformed in any way. Im not overfilling the master. My brakes work fine but the brake fluid comes out of the rear bowl of the master and drips down the back of the master, gets on the front of the booster and then drips off of the bottom of the master and booster all over my chassis and everything underneath. Any ideas? Id like to know if Marroan2 ever got his problem fixed if he is still on the fourm.
Drifting
Joined: Jul 2009
Posts: 1,532
Likes: 14
From: Dallas-Fort Worth TX
Guess... what... Mine is leaking to. Very slightly but its leaking. I ran my finger underneath the MC today and I had a few drops on the end of my finger. Its a new MC and new brakes all around. Brakes great, pedal is tight. I am really happy with the stopping performance so far. But the leak does concern me. I did notice that my new MC did have some edges underneath the cap. The cap clamps seem to be very tight. Although the leak is not bad enough to ever reach the ground. The area around the cap seem to have some reddish/rusty residue where Its leaking. The fluid level seems ok. Not sure how full your suppose have the MC. But both resevoirs look to be full.
1st Gear
Joined: Sep 2013
Posts: 1
Likes: 0
From: Fort myers Florida
Did you fix it ??
Guys, I realize this is an old thread but Im having the exact same problem. Ive replaced the master with a new Delco unit, still leaks at the cap. Ive used 3 different caps with 3 different cap gaskets and 3 sets of bails and it still leaks at the back of the cap closest to the booster. The mating surfaces of the gaskets and the top of the master are not deformed in any way. Im not overfilling the master. My brakes work fine but the brake fluid comes out of the rear bowl of the master and drips down the back of the master, gets on the front of the booster and then drips off of the bottom of the master and booster all over my chassis and everything underneath. Any ideas? Id like to know if Marroan2 ever got his problem fixed if he is still on the fourm.
Thank you amitchell4447@yahoo.com
