Brake fluid pushing out of master lid

[PDQ]

So I'm still chasing down my dragging brake issue with the pedal that won't quite return documented in the previous disc brake dragging post. I thought it may be from header heat causing pressure in the lines, however today I installed some aftermarket heat shields both on the hard line from the prop valve and also in front of the prop valve itself. (with an air gap). Same result. Starts off fine but as you drive for a bit and temps come up (assuming it's somehow related to temps) they start to drag as the pedal needs a slight pull back to fully release. I've ordered a new booster to try that, but the other issue that is throwing me off is as this is going on, fluid starts to seep out of the master cap to the point where its dripping on the fenderwell. Once everything cools, it all goes back to normal.

What's causing the fluid to push out of the master once it comes up to temp? I changed from manual drums to front power discs but no other mechanical mods.

help

 

[72Sport]

So I'm still chasing down my dragging brake issue with the pedal that won't quite return documented in the previous disc brake dragging post. I thought it may be from header heat causing pressure in the lines, however today I installed some aftermarket heat shields both on the hard line from the prop valve and also in front of the prop valve itself. (with an air gap). Same result. Starts off fine but as you drive for a bit and temps come up (assuming it's somehow related to temps) they start to drag as the pedal needs a slight pull back to fully release. I've ordered a new booster to try that, but the other issue that is throwing me off is as this is going on, fluid starts to seep out of the master cap to the point where its dripping on the fenderwell. Once everything cools, it all goes back to normal.

What's causing the fluid to push out of the master once it comes up to temp? I changed from manual drums to front power discs but no other mechanical mods.

help

There is an adjustment screw on the front of the shaft the presses on the piston on the master cyl. Tighten the adjustment screw 1/2 turn and put it back together. That should fix all your problems. If the booster shaft is too long the brake fluid can't return to the mc reservoir. Brakes start to drag. Brake fluid gets hot. Pressure builds up, hot brake fluid expands and returns to the mc reservoir and makes a mess. If that doesn't fix the problem tighten the adjustment screw another half a turn. This should make messing with the brake pedal unecessary.

 

[PDQ]

72, thanks for the reply
I have tried that. Originally the master would not sit flush on the booster. I ended up taking the lock nut off the adjusting tip which allowed the end to back all the way up to the rod, which then got me about 1/16th or better of a gap between the booster rod and the master which is also much as I can take out. The booster shaft tip is threaded all the way tight against the shaft at this point. I found that issue from your advise on the other thread...any other thoughts?

 

[72Sport]

72, thanks for the reply
I have tried that. Originally the master would not sit flush on the booster. I ended up taking the lock nut off the adjusting tip which allowed the end to back all the way up to the rod, which then got me about 1/16th or better of a gap between the booster rod and the master which is also much as I can take out. The booster shaft tip is threaded all the way tight against the shaft at this point. I found that issue from your advise on the other thread...any other thoughts?

Is the master cylinder for front discs? I can't be of much help. I have never had any of these problems.

 

[Slowleak]

Make sure your fluid level is not too high. You need to fill it to 1/4-1/2 inch from the top. Fluid is going to slosh around in there. You may have a bad seal or weak bale holding the lid on. Sometimes you have to reshape them a bit.....

 

[SteveL]

I had a mc that kept leaking from the cap. I tried the usual fixes and it still leaked. I ended up replacing the mc. Now it's fine.

 

[Apogee]

Odds are that the MC cap leaking has nothing to do with the brake issue you're experiencing, and is just a separate issue. When you apply the brakes, a squirt of fluid will hit the cap since that's the easiest place for the fluid to go until the seals cover the compensator ports in the bottom of the reservoir, the ones that allow fluid into the piston bore. Once those are covered by the seals as the pistons move forward, then when you release the brakes, the fluid flows back into the MC due to the calipers/wheel cylinders retracting/collapsing respectively, and you'll get another shot of fluid at the MC lid.

Overfilling the MC can certainly contribute to the issue, however most often it's just poor quality rubber gasket, rough sealing surfaces on the cast iron master cylinder, a weak bale (metal strap that holds the lid down) or some combination thereof.

I did just have a similar issue with my brakes dragging in my '77 F150 after not driving it much for a while, and it turned out to be the booster failing in a way I'd never seen before. The air valve was allowing the brakes to be applied slightly without any pedal input, but otherwise they seemed to work fine.

 

[Rustytruck]

When you apply the brakes the piston in the master cylinder seals off the port and compresses the brake fluid. The fluid pushes out the wheel pistons and applies the brakes. When you release the brakes the fluid pushed into the wheel pistons has to come back into the master cylinder. It squirts this fluid in small ports inside the master cylinder compartment. If you have air in the lines it must compress that air too and displace more fluid. If you have worn out shoes/pads the system will displace more fluid too. This extra fluid makes the fluid squirt harder and longer. Enough to squirt on the cap where it seeps to where the cap is not sealed properly. Take the cap off and look at the rubber and you will see a compressed indentation in the gasket. If the cap is distorted you will see areas that are not compressing tightly, those areas need adjustment to seal tightly and contain fluid seepage. Cheep offshore caps really suck and sometimes your better off using your old cap.

 

[Broncomt]

Make sure your fluid level is not too high. You need to fill it to 1/4-1/2 inch from the top. Fluid is going to slosh around in there. You may have a bad seal or weak bale holding the lid on. Sometimes you have to reshape them a bit.....

X2
Had same problem on 68 mustang when I put discs all around. Less fluid fixed it.

 

[PDQ]

Thanks all, some good points I hadn't thought of to follow up on. And I do often miss the simple fix first......

 

[darkmagus]

I kept having this issue and what I ended up doing is taking the rubber out of the cap and flipping it over so that the raised areas are now facing down towards the fluid. Hasn’t leaked since. I had already lowered the fluid level as far as I was comfortable with.