109 or 88 dual master cylinder?

[busboy]

If the bore on the front circuit portion of the 109 MC is a larger diameter than the bore on the rear circuit section, you will be moving more fluid to the front wheel cylinders than the rear cylinders for the same amount of pedal travel, actuating the front brakes before the rear, causing the front shoes to do more of the braking work than the rear.

I'm not saying that's a problem--but I am saying that it's probably happening on your 88 if the MC bores are different. Presuming you don't have the fronts adjusted way out, that is...

Yeh I could see it being a problem if it was the rear brakes getting the extra fluid but I think the front can take it. My brakes are like new and adjusted right up, I don't use it very much at hi-way speeds anymore so with no heavy braking they pretty well stay in good adjustment. Sitting thinking about it I did know that the front portion would produce more fluid when I installed it so I remember taking it extremely easy to see how it responded. Memory is not so good anymore.

[o2batsea]

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. So, assuming that the drums, shoes and wheel cylinders are in good condition and adjusted properly, an 88 MC would be perfectly fine and serviceable on a 109.
Also, the MC has a shuttle piston, so no, the fronts don't come on before the rears. All operate under equal pressure simultaneously. What happens when you brake is a weight shift from back to front, so that most of the actual stopping is done by the front wheels ( and why you don't jab the rear brake only when on a motorcycle). That's why you want them too be more stout. Rear wheels will lose braking traction and lock up before the front because there is less force acting on them. ABS works primarily on the rear axle for this very reason.

[TeriAnn]

So, assuming that the drums, shoes and wheel cylinders are in good condition and adjusted properly, an 88 MC would be perfectly fine and serviceable on a 109.

I beg to differ on that one. When I converted to power bakes I was accidentally shipped an 88 master cylinder. Not knowing the difference I installed it. No brakes or absolute minimum brakes the first pump. That was with all shoes adjusted one click down from full lock and dragging on the drum. Out 2 clicks from lock I needed 2 pumps to get any brakes.

[o2batsea]

I beg to differ on that one. When I converted to power bakes I was accidentally shipped an 88 master cylinder. Not knowing the difference I installed it. No brakes or absolute minimum brakes the first pump. That was with all shoes adjusted one click down from full lock and dragging on the drum. Out 2 clicks from lock I needed 2 pumps to get any brakes.

our experiences differ indeed.

[greenrover]

thanks TeriAnn

I took a look at your pictures. None of them are a eye level profile of the body but it looks to me like you are installing an 88 master cylinder. If you look at the master cylinder pictures I have about half way down the web page you can see the differences. The 88 brake cylinder body is a constant diameter when viewed from profile. The 109 master cylinder when viewed from the side is taller for the half of the cylinder body closest to the servo unit then about half way down its length steps down. This is the diagnostic visible difference between the two types. Look at my pictures then look at your master cylinder. It should become obvious.

TeriAnn
Thanks for the response.

I took another look from the side and the casting does change in diameter (see pic) and consistent with their pics the diameter of the rear port is larger (~7/16") than that of the front port (~3/8") so it seems it is a 109MC. So I think I got the right MC for my conversion. However when I was looking at the series III shop manual on how to run the lines I noticed the manual contradicts your web site as to what axle goes to what port. 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. Understand your logic about needing more volume for the front brakes but could it be possible that the fitting diameter does not correlate to bore volume displacement?
Can anyone tell me how the fluid flows inside the MC bore when it is stroked?SIII WS manual Brakes.pdf

[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.