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Master Cylinder Issues

Charlie Brown

Charlie Brown

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Fairly new (less than 2 k miles) on a manual four wheel disc conversion from Dr. Diff. Two issues.
1) - after car sits for a day or so, first application of brake pedal results in pedal going to the floor. Second application of brake pedals results in a normal pedal. Brakes then function as normal. No external leaks except for ........ read item #2
2) - brake fluid weeps / leaks at caps. Unit is not over filled.
Sent an Email to Cass at Dr Diff and he said probably air in the rear lines.
I'm not convinced this is the issue.
Master is 15/16 bore with plastic caps and I believe alloy body.
Anyone care to chime in on this?
 
Did you buy and install the Dorman replacement caps and seals that Cass recommends? I bought 4 aluminum master cylinders from him and they all leaked at the caps in short order. Knock on wood, since replacing the those parts, no more leaks.
 
hunt2elk said:
Did you buy and install the Dorman replacement caps and seals that Cass recommends? I bought 4 aluminum master cylinders from him and they all leaked at the caps in short order. Knock on wood, since replacing the those parts, no more leaks.
Click to expand...
I haven't replaced the caps - but looks like that could be a good option. My main concern is item #1 - pedal issues. Thanks for the cap tip.
 
Re-bleed your system. My car was the same. I did have air in 1/2 of a front caliper. Once the air was out rock hard pedal all the time.
 
I have the same problem. I have bled system several times to no avail
 
Air in the nose of the master. Only way you'll get it out, without taking it out of the car to bench bleed.
beerestoration2018 2085.JPG

beerestoration2018 2083.JPG
 
dadsbee said:
Air in the nose of the master. Only way you'll get it out, without taking it out of the car to bench bleed.
View attachment 1120690
Click to expand...
That can be true. Common mistake a lot of people make when bench bleeding. It takes extra effort to press both pistons which most people unintentionally make.
 
Here’s what I did and it’s worked
I noticed the corvette master cylinder on my 73 trans am has a small little hole on the metal cap, the master cylinder in my 69 camaro also has a small vent hole in the cap so I added one to the satellite

my satellite has a brake conversion for the front since it was drums before anyway I always felt air in the system all the time and the master cylinder plastic caps would leaks like you described especially after a drive. It actually mest up the paint on the inner fender.

I made a really really small hole on the top of each cap with the smallest drill bit I could find and since then I don’t feel any air in the system and the leak stopped even after a hard drive
 
Get the Dorman replacement caps to fix the leak. And rebleed the master cylinder and rebleed the brakes. Air in line is what causes the "pump up the brakes on start up" problem. I had the exact same problems after converting to discs on my 69 Roadrunner. I felt like I bled the brakes about 30 different times. I tried vacuum, pressure, gravity. having the wife pump the brakes. Finally did it with a hose going from each rear bleeder individually to a half filled bottle on the floor and pumped the pedal about 50 times. Finally worked. I also had to get an adjustable master cylinder push rod to bring the pedal up a little higher. You can get it working correctly. It just takes a lot of perseverance.
 
Here are the caps and gaskets you need. Agree with the others that you still have an air pocket in the system.

20210607_062251.jpg
 
Charlie Brown said:
Fairly new (less than 2 k miles) on a manual four wheel disc conversion from Dr. Diff. Two issues.
1) - after car sits for a day or so, first application of brake pedal results in pedal going to the floor. Second application of brake pedals results in a normal pedal. Brakes then function as normal. No external leaks except for ........ read item #2
2) - brake fluid weeps / leaks at caps. Unit is not over filled.
Sent an Email to Cass at Dr Diff and he said probably air in the rear lines.
I'm not convinced this is the issue.
Master is 15/16 bore with plastic caps and I believe alloy body.
Anyone care to chime in on this?
Click to expand...

I have the same problem here too. Intially: Bench bled (with syringe back bleeding the master), installed, bled regular brakes. Solid and good pedal. Installed motor which wiped cam immediately. Car sat for a year while I organized a different motor trans combo. Finished that, it all went good, installed and started driving car and now I have the exact same problem that you mention.

I guesss I’ll try bleeding four corners again first, then the master again if I have too... but was also wondering if maybe fluid was bleeding by seals in the master or something.

No leaks in the system anywhere..... I have the same master 15/16 bore
 
4mulas said:
I have the same problem here too. Intially: Bench bled (with syringe back bleeding the master), installed, bled regular brakes. Solid and good pedal. Installed motor which wiped cam immediately. Car sat for a year while I organized a different motor trans combo. Finished that, it all went good, installed and started driving car and now I have the exact same problem that you mention.

I guesss I’ll try bleeding four corners again first, then the master again if I have too... but was also wondering if maybe fluid was bleeding by seals in the master or something.

No leaks in the system anywhere..... I have the same master 15/16 bore
Click to expand...
I have a very trusted old timer mechanic who seems to think the same as you about fluid leaking past the seal. I will, however, re-bleed the entire system to see if this makes a difference.
 
I can't remember exactly, but I thought Cass told me once that if the brake pedal is spongy on the first pump after the car has set for a day or so but then normal after that, the master could be leaking internally.
 
Charlie Brown said:
I have a very trusted old timer mechanic who seems to think the same as you about fluid leaking past the seal. I will, however, re-bleed the entire system to see if this makes a difference.
Click to expand...

Check your rear brake adjustment before re-bleeding. If they are too far away it can create this problem too
 
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