From Rick Ehrenberg: Mopar Action Tech questions Issue: 10/07 Vol 16 Number 8
Customer Wrote: I did away with my Vacuum booster. 440 engine. It's a big car and has a hard solid pedal feel now. Question: what is the p/n or application for a 7/8" bore Mopar master cylinder? It needs to be the older 4 bolt cast iron master cyclinder.
answer:
Tom you're right. either the pedal ratio (hard to change) or the hydraulic ratio (easy to change) is the fix. It's a give and take proposition to fix. As the pedal pressure required goes down, the pedal travel goes up. there's no free ride! That's why as much as I dislike em on a 4,000 lb car, a power booster is usually required or at least desired. there was a 15/16" bore unit used on a drum brake A bodies (Bendix P/N 11516) Now normally a drum master can't be used for 2 reasons: First is it has a front-outlet residual pressure valve, which would cause the front discs to drag like mad. Luckily this is easy way to remove. Then there's the smaller resivior size, which means you could run out of brake fluid before the pads have worn out. However, the online photos I have seen of this Bendix replacement show it having the larger "disc" resiviors. This same unit is shown for D100/150 pick-ups. Volares, early 70's B Bodies, etc., all with manual discs, so it may not even have the residual pressure valve.
Going from 1" to 15/16" may not sound like much, but it will result in 12 percent less pedal pressure, which will be noticable. If there's a large Mopar 7/8" unit out there, I'm not aware of it. Plus, many of the smaller brand X one's I've seen do not have the machined recess for the manual pushrod retainer. However, it may be out there somewhere, if a reader hooks me up with the ratio, I'll pass it along.
