Cardone MC questions

[jjbunn]

The technical article about the Ford Courier Master Cylinder is great!

I have the Cardone 13-1777 MC, which is the specified model, and I've been comparing it with the original S2 MC on the bench. In the tech article, Joe says:

"The second and most critical issue is the Girling OEM pushrod arrangement. As is, the OEM pushrod arrangement will physically fit into the bore and work with the Cardone but the primary piston of the Cardone does not have the matching hemispherical recess to accept the head of the Girling pushrod. This will cause the piston to be pushed in a few millimeters when the Girling pushrod is locked into position with the circlip into the Cardone. In effect, all pedal free play is lost and in my case, brakes started locking up within 2 miles of a test drive."

On the Cardone unit I have, putting the stock pushrod into the bore and securing it with the circlip does indeed push the piston slightly inwards, maybe a few millimetres. Why does this cause the brakes to lock up after a while? Why is free play required - does something expand in the MC on use?

Thanks!

[LotusJoe]

I actually only published that article. It is written by Grumblebuns (Joji). But I think what is going on is with the piston is slightly pressed the pressure is never released. So very time you press the brake pedal the pressure is increased. Eventually the brakes are locked up. The piston needs to completely relax to release the pressure. I'm sure Grumblebuns will jump in here and correct me. 

[jbcollier]

Brake seals/cups allow the one-way flow of fluid: ie. fluid into the system but not out.  There is a small pressure compensating port that is only exposed when the piston is fully returned. This port allows system brake fluid access to the reservoir.  Brake fluid doesn't compress but it does change in volume when its temperature changes.  As the brakes heat up, so does the fluid.  This increases the fluid's volume and would apply the brakes, unless this excess volume/pressure is relieved when the pedal is fully released.  Thus the need for free play when the pedal is fully released.

[Grumblebuns]

John is correct. This link explains how the master cylinder works. The brakes will eventually lock up from fluid volume expansion if you try to use the OEM pushrod arrangement on the Cardone.

http://www.tegger.com/hondafaq/mastercylinderreplace/howworks.html

[jjbunn]

Thanks very much for the explanations, much appreciated.