Repeat M/C leak from reservoir lid

[DirtyLarry]

Alright, I am scratching my head on this one. The master cylinder lid started leaking all over the freshly painted inner fenders on my ’72 C20 a while back so I picked up a new lid seal and called it good. A few days later I noticed it was leaking again so I pull off the two lid retaining springs and bent the ends over a bit to help hold the lid on tighter. I figured all would be well….nope! It started leaking again so I bought a new master cylinder from Vhatozone and ran that for a few weeks. Guess, what? The SOB is still leaking! It leaks from the lid at the forward reservoir, which would be for the rear brakes.

I don’t know what do to next other than replace the rear brake hose and maybe wheel cylinders.

Anybody ran into this before? The funny thing is the brakes work flawlessly. It is just the more you drive it the more it leaks from the lid as if it is building pressure that won't release.

The truck is for sale but I want to fix the brakes before I sell it to someone with a brake problem. The truck looks rather rough but it is in perfect running order except for this leak.

[tcb-1]

just playing the devil's advocate here, but are you overfilling? I did that a couple of times with the same result.

also, is the bladder "sucked" out when you open it?

[DirtyLarry]

No, not over filling it. Just the same amount of fluid in this truck as any other I have ever had. It just started leaking one day....and has never stopped since. Frustrating.

[tcb-1]

I'm then going to have to defer to the real experts on here.

one thought though. I'm with you on thinking it may be the rear wheel cylinders. if your over pressurizing them that may subsequently cause a backup of pressure into the master cylinder.

BUT, I'm no expert. just thinking out loud......

[DirtyLarry]

Nobody else has any ideas, eh?

Where are all the members with a million reputation points that are supposed to be the village experts on anything 67-72 related?

[Green Machine]

Does the front reservoir seem to get fuller on fluid, like it might be moving from the rear half to the front ? Only other thing I can think think of is that is the leak might be somewhere else, like the prop valve or a line ?

[jaros44sr]

Maybe the prop. valve is closed to the front wheel brake lines and all the fluid is going to the front chamber...don't know just a guess. Seems strange your brakes are working good

[stevewhitmore]

Not an expert here, but I had a similar problem on my 67 Camaro and the vacuum hose had come off at the manifold

[DirtyLarry]

It is a weird deal. The fluid is not moving from one reservoir to the other (the rear reservoir is maintaining level while the front is being pushed out of the lid) and I didn’t recently top it off before it started leaking. The vacuum source is fine and the brakes actually work just as good as they always did. It just seems to push fluid out of the lid once it is driven for a while, which leads me to think the rear brake hose is goofed up inside. One of the first things I looked at was the prop valve to see if it came off center. From what I can tell it is still centered and I get all four wheels to lock up when hammering the brakes in the dirt. Craziest thing. Even more crazy it is just started one day out of the blue. No fluid was added, no brake pads or shoes changed, nothing…..

[BLACK AND BLUE 67-72]

I had the same problem with 4 master cylinders from Autozone, all of them leaked except the 5th one I got.

[Blazer1970]

My old master cylinder did that same thing. Replacing the master cylinder fixed it. Try another brand of M/C if you can.

[Woody]

I have had this problem before as well. I have cleaned the top lip of the reservoir (it was slightly rusty) to remove the crap and it sealed. it is leaking again but this worked for a year or more.

Also you may have resurface the top lip of the reservoirs to get a better seal. To do this you need to empty most of the brake fluid from the reservoirs and then stuff them with a rag or paper towel. Use a flat file or sandpaper wrapped around a flat piece of wood and file/sand the top until smooth. Clean up the mess you made. Next make sure the MC lid to straight by laying it on a flat surface. Repair as needed. Remove the rudder gasket and ensure it is clean on both sides. Then refill MC with brake fluid and replace the lid.

That should fix your problem. Good Luck.

[LockDoc]

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Check and make sure none of the brake lines that run to the rear brakes are close to the exhaust pipes or mufflers. Excessive heat can make the brake fluid expand and force it out of the master cylinder. If you find any spots that are suspect and you can't re-route them split a piece of rubber hose and put over the brake line and see if that helps. A loose fit is better than a tight fit on the hose.

LockDoc

[leddzepp]

Subscribed! I have a 72 C-20 with the EXACT same problem.

[rlm316]

In my opinion, I don't think that it is the rear wheel cylinders or the flex line. Wheel cylinders won't increase pressure on the system, they would blow the end seals and leak, and if the flex line collapsed it would either hold line pressure on the wheel cylinders causing the brakes to drag or blocking pressure from reaching the cylinders resulting in loss of breaking to the rear. You have not told us if the fluid level is dropping and from what bowl. Your brake system only holds a designated ammount of fluid, so for it to increase/expand it must either be heated or bled into from another system/circuit. When you pull the cap, is there a difference in level in either of the bowls?

[DirtyLarry]

Originally Posted by rlm316

In my opinion, I don't think that it is the rear wheel cylinders or the flex line. Wheel cylinders won't increase pressure on the system, they would blow the end seals and leak, and if the flex line collapsed it would either hold line pressure on the wheel cylinders causing the brakes to drag or blocking pressure from reaching the cylinders resulting in loss of breaking to the rear. You have not told us if the fluid level is dropping and from what bowl. Your brake system only holds a designated ammount of fluid, so for it to increase/expand it must either be heated or bled into from another system/circuit. When you pull the cap, is there a difference in level in either of the bowls?

Yeah, I did….very first post, first sentence, last line.

It leaks from the lid at the forward reservoir, which would be for the rear brakes.

Only the front bowl is leaking. The rear bowl is maintaining a steady amount of fluid. I haven’t added any fluid to the front bowl before or since this leak began other than when I replace the m/c. I was actually hoping I over filled it and it would find its sweet spot which it hasn’t found yet so no luck there.

It would still seem odd to be heat related because nothing changed. It just started leaking out of the blue. I won’t have time to crawl all over it for a few weeks. I appreciate all the ideas though. Keep them coming!

[Hawker7]

Did you ever get this fixed? Mine is doing the same thing and I'm nervous to put my new painted inner fender in.

[mclairmo]

I don't think pushing on the master cylinder piston would force fluid out of the top of the master even if the hoses and lines are plugged. Seems to me that the fluid is being forced back from the rear wheel cylinders and into the master, raising the level and causing the leaking. Or, the lid is not sealing all the way around and is sloshing out as the truck moves around. I would make sure that the rear shoes are not retracting too much for some odd reason, thereby pushing fluid back into the master. Also, try filling the master only half way. Also, make sure the sealing surface is perfectly flat with no nicks. Finally, try a master cylinder with a bolt on lid. New replacements sometimes have a bolt in center of the lid that retains it.

[71tahoe]

The only time I've seen one overflow was from transfer from the other reservior so I'm not going to be much help.

Is there a difference in a half and 3/4 ton? I have 70 and 71 half tons and unless got crossed up tracing the lines my front reservior goes to the front brakes.

[A.P.]

I had the same problem, my solution was replacing the M/C, calipers, rotors, and pads.

[chipflyer]

I had the same issue plus I could never get the front brakes to bleed and the brake light would not go out. My MC was a brand new one from Autozone, and after trying a different wheel cylinders in back and a new/different prop valve I gave up and took it to Midas to have the brakes power bled. $30 and 2 blocks later the light came back on.......
I went to NAPA and bought a rebuilt MC, easily bled the brakes and the leak stopped also. No trouble ever since.
I would try another MC.

Jeff

[hvychve]

well, if I am understanding you correctly, its leaking out from the lid, if thats the case, I had one many moons ago , do the same thing, if its got the wire straps on top that hold the lid down, than, remove the straps and tweek em a bit to allow you to pull lid down tighter, If memory serves me correctly I bent it right where they push into the holes of master, bend em up and thus this will cause lid to be more snug. good luck

[Indyuke]

I had the same problem once. I did the same thing woody did, which was resurface the top of the MC to remove any irregularities. Doesn't leak a drop now.

It does leak internally though, from the rear res to the front res. Bad seals maybe? Someday I'll have to replace the MC.

[70's luvr]

I ended up doubling the gaskets and no more leak.

[Andy4639]

I just worked on mine today. I had problems since March with mine as you it just started one day which for me was a 5 hr ride home. I took some rubber gasket material and cut it out to fit the top around the old gasket. I cut it so the orignle rubber gasket can work as it should. The rings in the rubber are for vacuum in the system to take up the space when you mash the pedal and then collasp back when the pedal is released. I cut my extra gasket with the holes for this and extra around the lid. I haven't drove it yet since I'm doing some other mods to but hope this fixes my leak.