Rebuilt Master Cylinder/Bench Bleeding Question

[mattimuss]

Hey All,

 

I'm having a "Gibb's Moment"...my gut is telling me that something is not right...

 

Info:

I found two OEM rear wheel cylinders on eBay.  I pulled them apart and they look good, no pits and the rubber is supple.  I am having White Post Restorations rebuild the front cylinders.  I have made all new brake lines.  I had the rear shoes relined [due to rear end lube leaking all over them] and the fronts are in very good shape.  I will be having the drums/hubs cut and the shoes arched.  I had the master cylinder rebuilt.

 

I searched the forum and read all I could find on brake jobs. Numerous posts by members, far more knowledgeable about these classics than me, advised bench bleeding the master cylinder.

 

Now for the "Gibb's Moment":

I made a bleeding line and connected it to the master cylinder, filled the master cylinder with fluid, and started gently pumping the piston.  Everything was going good and I was starting to get pumps with no air bubbles coming out of the bleeding line.  I noticed that the piston was getting harder to push in and it was taking a long time [close to ten seconds] to come all the way back.  This is causing the bad feeling.  In the begging the piston went in easily and came right back.  Now it is hard to push in and it takes a long time to come back.  I am not liking this!

 

So, did I screw up the freshly rebuilt master cylinder?

[mattimuss]

Hey All,

 

I disassembled the master cylinder this evening and found the piston completely surrounded by brake fluid.  This does not seem right to me.

 

My thought is that I may have pushed the piston in too far while bench bleeding the master cylinder, maybe pushing it past both the port and relief port which allowed fluid to encase the piston.  Could this be correct?

 

I thoroughly cleaned all the parts and will be reassembling it tomorrow with a new top gasket [i have to make one].  Is there a limit to the distance I should push the piston in while bench bleeding?

 

As always, any help suggestions are appreciated.

[Dave72dt]

You should have fluid on both sides of the piston and none coming out the end by the pushrod. Remember you have those ports that fluid has to come out of to release the brakes. Revisit the schematics of a master cylinder and fluid movement through it. The small port hole may be partially blocked slowing the fluid return and you only have 1 spring assisting the return of the piston. When in operation, you also have the wheel cylinder return springs to assist you plus a pedal return spring on the frame.

[mattimuss]

Hey Dave, thanks for replying! 

 

I completely disassembled the M/C yesterday.  I ran 80psi of air through both port holes and then shined a flashlight into them to make sure they were unobstructed...all looked good.

I'm going to reassemble it tonight and test it again.