MASTER CYLINDER QUESTION

[CFD244]

Hi Folks

1971 Demon with power K/H 4 piston calipers....All stock. My manual says that the bore for the OEM master is 1 1/32". I see a lot of reference about using a 15/16 bore master. What benefit would be gained on a completely stock system? Thanks.

 

[mvh]

The smaller bore reduces pedal effort in exchange for increasing pedal travel. But the power brake master cylinder should already be 15/16 — are you sure you are reading the table correctly? It's the manual brakes that had the 1-1/32 bore, and they are famous for an extremely stiff pedal. The power brakes got the smaller bore (contrary to apparent logic) because of the lesser pedal leverage caused by the presence of the booster.

 

[CFD244]

The master that I pulled from the booster had the 1 1/32 bore, but that's not to say that the master hadn't been changed at one point in it's life. My manual simply said 1 1/32 for K/H. It also looked like most other models used this bore as well.

 

[mvh]

Actually, you are correct — they didn't use the 15/16 MC for power discs until 1973 — I just looked it up. Through 72 they were all 1-1/32. I wouldn't change the MC bore on a power brake system — you don't know what effect that would have on the booster (for example, what is the effect of a greater range of travel?). And since the brakes are already boosted, I can't see why you would want less pedal effort. It might be a consideration if you were thinking about converting to manual brakes.