109 or 88 dual master cylinder?

[greenmeanie]

If your brake shoes are adjusted properly, the amount of travel is a matter of a hundredths or even thousandths of an inch. The amount of fluid required to push the wheel cylinders open this much is very small. That's the very definition of a firm pedal.

Thats incorrect. The distance the shoes travel is dictated by the travel of the piston in the hydraulic cylinders. This travel is a function of fluid volume moved. The distance the shoes travel manifests itself as pedal travel.

The firmness of the pedal is a function of the compressibility of the working fluid in the sytem and its ability to apply force to the shoes once they are in contact with the drum. THAT is the definition of a firm pedal.

A 109 has more fluid volume in the front cylinders than the rear. To shift the shoes the same amount (Assumes well adjusted brakes) you need a larger volume in the bore closest to the pedal. The pressure of the fluid controls the flow rate at which the pistons move before contact and then the final force applied to the brake shoe once it is in contact.

The theory behind hydraulics is not that difficult. It would seem that 99% of all automotive manufacturers seem to follow the same set of rules about bigger brakes and the resulting bore size difference in the MC for the fronts vs rears. One exception is the 88 with its front andrear 10" drums. I agree with TAW on this one.

[o2batsea]

Why so testy? Did I write something to get you mad?

[SafeAirOne]

[TeriAnn]

The manual says on a 109 dual MC the rear port goes to the rear wheels and the front port goes to the front wheels and that it is the opposite on the 88 MC.

I think the confusion is that the terms front and rear are in respect to something.

When I write rear I'm thinking in respect to the vehicle it is installed in which makes the rear the part closest to the servo unit.

I believe the manual is writing about the master cylinder in respect to itself with the front being the opening where the servo plunger presses into the first master cylinder section and the rear being the section that is farthest from the open end. In respect to the cylinder end opening what the manual calls front I call rear.

You can always tell someone who has taken too many physics classes by asking which way is up. They will likely answer with another question: "in respect to what?" Up & down, Forward & reverse, front & rear are all relative terms that require a point that they are in reference to.

[greenrover]

I think the confusion is that the terms front and rear are in respect to something.

When I write rear I'm thinking in respect to the vehicle it is installed in which makes the rear the part closest to the servo unit.

I believe the manual is writing about the master cylinder in respect to itself with the front being the opening where the servo plunger presses into the first master cylinder section and the rear being the section that is farthest from the open end. In respect to the cylinder end opening what the manual calls front I call rear.

You can always tell someone who has taken too many physics classes by asking which way is up. They will likely answer with another question: "in respect to what?" Up & down, Forward & reverse, front & rear are all relative terms that require a point that they are in reference to.

Found this site http://www.buckeyetriumphs.org/techn...ory/Theory.htm that discusses how things work inside the dual MC on a TR6 but I think it applies to the 109 dual circuit as well. I also read some place that there may be a difference between MC with roll pin versus bolt on reseviors - do you guys think that is true. Mine is a bolt on like the TR6 so based on what I have read the front brakes should connect to the larger port located closer to the bulk head. Which means I need to talk to RN cause the brake line fittings to go to rear wheels is too big.