Issue with early Tokico brake master cylinder

[larryw]

I can't get a decent pedal. I have a series 1 car (#20334) with the Tokico m/c that uses the push in reservoir caps and the large (front) reservoir to the front. All wheel cylinders have been disassembled, cleaned and rebuilt. Piston/pucks move freely, there are no leaks. Master cylinder has been cleaned and rebuilt with a genuine Tokico major rebuild kit that included all the internal parts, not just the rubber bits. The interior bore is clean with no pits or gouges. The design is unique. There is no solid connection between the front and rear pistons. There is a spring between them but the actual braking force seems to be transmitted by a slug of fluid between the two. I have good rear brakes but I can't seem to get the fronts to fully come on. There is no air in the system. Sitting in the seat and pressing the pedal, I can feel fluid bypassing internally and I think it's the slug of fluid that's supposed to be pushing the front piston forward. I'm using a DOT 4 fluid. Could it be the DOT 4 isn't swelling the rubbers as much DOT 3 does? I have no seeps anywhere else. The clutch circuit works fine with the DOT 4. I'm going to suck out the reservoirs and refill with DOT 3 and rebleed the master to draw it inside. If that doesn't fix the problem I've got to get a new master, unless someone's got another solution. Is there some other trick to these things? Were they just prone to this problem? I intend to run the stock disc/drum system, whats the recommended replacement master cylinder, if it comes to that?

Thanks.

[jfa.series1]

Your description suggests there is a leak between the two pistons. Presuming you have the rubber cups installed in the correct direction AND there really is a slug of fliud between them (how does it get in there?), the leak may be in the bore surface. Did you run a brake hone thru the bore to eliminate any minor surface blem issues? My wheel cylinders looked good when I rebuilt them but I still had leaks until I went back in with a hone.

Good luck on keeping your Series 1 running, we need to keep all them out there to "show the flag"!!!

Jim

[beermanpete]

The design you describe is typical of all the 2-circuit master cylinders I have worked on. The "slug" of fluid is simply the hydraulic pressure that is generated by pressing the brake pedal.

Based on your description your master is bad. The only other option is a leak somewhere else in the system, but this would be revealed by brake fluid running out onto your car and/or the ground.

[larryw]

Well gentle readers, it turns out my rebuilt master cylinder was fine. There was a little "trick". The problem was within the booster. The "reaction disc" had become dislodged and was found loose within the booster. I reattached with a bit of crazy glue, reassembled everything and bled the brakes again. All fixed. Pedal nice and high, brakes work great!

See the attached thread borrowed from Hybrid Z: http://forums.hybridz.org/index.php/topic/69706-reaction-disk-pictures-and-walkthrough/

Maybe this should be a sticky?

Edited September 11, 2011 by larryw

[jfa.series1]

Great detective work. Congratulations!

Jim