Master Cylinder - Cover Leaks

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RichardJ916

RichardJ916

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I am in need of a new master cylinder because it is contenting leaking out the front, also I have to pump up the breaks all the time to build up pressure, and I am sure it is because of the leak, the front will run dry about one a year. It has front Disk and drum rears manual brakes. Can anyone please help me decide what the replacement part to order, I also would like to know is the smaller bore ones best for Manual Disk - Drum brakes

master cylinder.jpg
 
if it's leaking around the cap at the top,it's usually because of a rough casting. At the shop where I work,we always flat sand the master cylinder upside down-we place a piece of tan 80 grit 3m sandpaper rough side up on our flat bench,then with NO fluid in the master, sand it until it's smooth. It's amazing how rough the current crop of cast iron masters are on top. Once it's sanded flat , we carefully wipe any residuals off it while it's still open side down and then flip it over,bench bleed it like normal, put it on the car and re-bleed all the brakes. Note that we only do this with problem masters or new master cylinders. if it's leaking at the back,there is a problem with the seals and needs replacement. You can get a spongy brake pedal if you would lose all your brake fluid during normal operation and induce air into the system-i.e. if the top casting is rough.and each time you hit the brake the brake fluid squirts at the inside of the cap from the compensating ports and it slowly leaks out you could get a problem once all the fluid is out of that chamber. Since you have manual brakes, look inside the car by the brake pedal and see if it's wet by the pushrod=if it's wet there,the back seal is bad and time for new. Hope this helps
 
I have looked inside the car and no there is no fluid inside the car nor is there any leaking inside the rear or front brake area, just in the front of the cylinder. When I pump them up I can go out and there is a leak in the front only and go back inside the car and have kinda of weak pedal. Maybe I should take it off and sand down the top, is there any way of doing the sanding while still on the car, maybe drain the cylinder stuff rags and them do a master cylinder bleeding...
 
I just put on a new one Friday for the same reason on a 72 Dart manual brakes. Made my own plastic gasket to hold the pushrod cover also.

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Was yours leaking on the cover also, Your cover is a bit different, mine is Disk Front and Drum Rear. Maybe that is why the different cover. Mine is not leaking from the rear only on the top of the cover in the front only.
 
Yes my cover was leaking in the front. But no other leaks in the system
 
If I need to buy a new cover and gasket to fit better I have seen so many ones for sale on ebay that end up being a bit smaller than the one I have, or the 2 bumps on the top are reversed. I need the one with the straight front and the curved part on the rear. I have read also the Ford makes the same type but it still is not the correct size, and I hate to buy the whole cylinder just to get the right cap. I have measure the cap and none seem to be the correct size.
 
OL DODGE said:
if it's leaking around the cap at the top,it's usually because of a rough casting. At the shop where I work,we always flat sand the master cylinder upside down-we place a piece of tan 80 grit 3m sandpaper rough side up on our flat bench,then with NO fluid in the master, sand it until it's smooth. It's amazing how rough the current crop of cast iron masters are on top. Once it's sanded flat , we carefully wipe any residuals off it while it's still open side down and then flip it over,bench bleed it like normal, put it on the car and re-bleed all the brakes. Note that we only do this with problem masters or new master cylinders. if it's leaking at the back,there is a problem with the seals and needs replacement. You can get a spongy brake pedal if you would lose all your brake fluid during normal operation and induce air into the system-i.e. if the top casting is rough.and each time you hit the brake the brake fluid squirts at the inside of the cap from the compensating ports and it slowly leaks out you could get a problem once all the fluid is out of that chamber. Since you have manual brakes, look inside the car by the brake pedal and see if it's wet by the pushrod=if it's wet there,the back seal is bad and time for new. Hope this helps
Click to expand...
If I have to order a new one, what kind should I get, Remember I have Disk Front - Rear Drum, I was told this was one from a 1973 that had Manual with disk front and drum rear.
 
Don't know if you want to try what I do, but it works . I sanded the master on a flat surface , I thought I did a good job but low and behold it still leaked out the front of the master , so what I do now is take the top off clean the rubber gasket with brake clean then clean the top of the master after you have it all nice and clean I take some automotive silicone and put a fine film on top when you have a fine film all the way around , replace the top. Works for me. With all these aftermarket tops hard to find a good one.
 
fatoldguy said:
Don't know if you want to try what I do, but it works . I sanded the master on a flat surface , I thought I did a good job but low and behold it still leaked out the front of the master , so what I do now is take the top off clean the rubber gasket with brake clean then clean the top of the master after you have it all nice and clean I take some
Click to expand...
Tell me more
 
fatoldguy said:
Don't know if you want to try what I do, but it works . I sanded the master on a flat surface , I thought I did a good job but low and behold it still leaked out the front of the master , so what I do now is take the top off clean the rubber gasket with brake clean then clean the top of the master after you have it all nice and clean I take some automotive silicone and put a fine film on top when you have a fine film all the way around , replace the top. Works for me. With all these aftermarket tops hard to find a good one.
Click to expand...
Between the rubber gasket and the brass top or between the rubber gasket and the top of the master cylinder edge...
 
RichardJ916 said:
Between the rubber gasket and the brass top or between the rubber gasket and the top of the master cylinder edge...
Click to expand...
RichardJ916 said:
Between the rubber gasket and the brass top or between the rubber gasket and the top of the master cylinder edge...
Click to expand...
I even thought about making a thicker rubber gasket between the one that goes there and the top of the cap, like adding a second gasket just around the edge and not making contact with the inter parts of the top
 
RichardJ916 said:
I even thought about making a thicker rubber gasket between the one that goes there and the top of the cap, like adding a second gasket just around the edge and not making contact with the inter parts of the top
Click to expand...
Suppose it's worth a try.
 
Never had a problem , when you remove the top in the future you have to scrape the fine film with a straight edge knife , brake fluid does not cut this stuff.
 
You likely still have air in the brakes. When you push the pedal, the air in the system compresses, thus the poor pedal. When you release the pedal, depending on the amount of air, a small burp, up to a large violent fountain of fluid can be propelled back into the M/C. The cap isn't designed to contain fluid shot at it. thus fluid migrates under the seal..

You can verify by having someone pump the brake pedal a few times WITH THE CAP ON, have the pedal held down, remove the cap (but keep it fairly close over to control the returned fluid) and have the pedal released. Whichever system shoots fluid back, (front, rear, or both) needs bleeding..

CAUTION.. Brake fluid is excellent paint remover and needs to be rinsed of any paintwork IMMEDEIATELY with lotsa water ! !

I often wet the whole area with water to prevent paint damage by any fluid spray..

hope it helps..cheers
 
inertia said:
You likely still have air in the brakes. When you push the pedal, the air in the system compresses, thus the poor pedal. When you release the pedal, depending on the amount of air, a small burp, up to a large violent fountain of fluid can be propelled back into the M/C. The cap isn't designed to contain fluid shot at it. thus fluid migrates under the seal..

You can verify by having someone pump the brake pedal a few times WITH THE CAP ON, have the pedal held down, remove the cap (but keep it fairly close over to control the returned fluid) and have the pedal released. Whichever system shoots fluid back, (front, rear, or both) needs bleeding..

CAUTION.. Brake fluid is excellent paint remover and needs to be rinsed of any paintwork IMMEDEIATELY with lotsa water ! !

I often wet the whole area with water to prevent paint damage by any fluid spray..

hope it helps..cheers
Click to expand...
I truly understand what you are saying, I will have time over the weekend to try this, thanks guys for you inputs to helping me figure this out...
 
If the brake fluid dissolves that black silicone (RTV) you will have either dissolved bits of RTV all throughout your brake lines and wheel cylinders, or at least have contaminated fluid. I do not think it's a very good idea to use something like that in this area!
 
69_340_GTS said:
If the brake fluid dissolves that black silicone (RTV) you will have either dissolved bits of RTV all throughout your brake lines and wheel cylinders, or at least have contaminated fluid. I do not think it's a very good idea to use something like that in this area!
Click to expand...
I understand, I will try to find a better way to seal the top cap to stop the leaking...
 
Flat-file the gasket surface, replace the rubber gasket with a softer more pliable one and increase the spring tension by tweaking the bale.
 
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