'66 Master Cylinder - Manual Brakes No Fluid Flow When Disconnected

[David P.]

When the brake line is disconnected from the master cylinder, no fluid is flowing out. I could leave it overnight and the master cylinder would not be empty. I rebuilt the master cylinder several times. Master cylinder has been bench bled. Piston is returning fully. It appears that, at rest, the piston is positioned in the bore where approximately the forward half of inlet port is covered by the front of the piston. So, fluid is obviously entering the area behind the piston. Does fluid enter the area in front of the piston through the holes in the piston and around the seal with air escaping through compensating port? Once fluid fills that area, shouldn't fluid just drain out the front of the master cylinder? I can't figure out what the issue is.

[Kurt K.]

Re: '66 Master Cylinder - Manual Brakes No Fluid Flow When Disconnected

Relaxed on the car or the bench, the piston should be back from the port so the fluid should flow out to the brake lines. When the piston is pushed in, the piston seal will move ahead of the port and block it off. Something is not right if the piston seal is forward of the port when you assemble it.

[Larry E.]

Re: '66 Master Cylinder - Manual Brakes No Fluid Flow When Disconnected

David: Just curious>are you using DOT#3 or Dot#5? Larry

[David P.]

Re: '66 Master Cylinder - Manual Brakes No Fluid Flow When Disconnected

The piston, being shaped like an hour glass, it appears that the front portion of piston is covering front half of inlet port. Piston is all the back against snap ring. No other way to assemble the master. I’m using Dot 5.

[David P.]

Re: '66 Master Cylinder - Manual Brakes No Fluid Flow When Disconnected

UPDATE- I believe I have an answer to the problem after talking to Lonestar. It appears that a lot of the reproduction master cylinders have a percolating/compensating port that is too small. I can barely get a strand of copper wire into the hole. That was my initial suspicion, but I wasn’t sure how big that hole was suppose to be. They told me I could go an 1/8” and it would be fine. I think that’s why you hear so many complaints about bleeding brakes. If you try to gravity bleed it would be next to impossible. Just make sure you hone the bore to eliminate any burrs. I’m going to attempt the fix and will report back here.

[Kurt K.]

Re: '66 Master Cylinder - Manual Brakes No Fluid Flow When Disconnected

I'm not familiar with the 66 Master but it should have two openings, inlet and compensating port. The smaller port to the front of the car (compensating port) should be wide open with no push on brake pedal. At rest, if that front piston seal is blocking or ahead of that port, no fluid will flow, and it could cause your brakes to apply as things heat up. Using Dot 5 may have swelled the piston seals to the point that they could be blocking that port. There is plenty to read about Dot 5 and how it effects some seals.

[David P.]

Re: '66 Master Cylinder - Manual Brakes No Fluid Flow When Disconnected

IMG_2743.jpgHere’s a picture of the port. Notice how tiny it is. That will also cause fluid not to flow.

[Patrick B.]

Re: '66 Master Cylinder - Manual Brakes No Fluid Flow When Disconnected

That is exactly the problem. I bought a reproduction 67 master cylinder and I could not bench bleed it. I noticed that the same kind of port you showed was smaller and longer, missing a counter bore that the original cylinder had. I had my original cylinder sleeved and it worked fine because it had the original GM style port. I was also using silicone fluid.

[David P.]