10" vacuum at idle - enough for power brakes?
 

Right now I'm running manual brakes with discs at all 4 corners. I have a 1.125" Wilwood master (it was their recommendation after I told them I was going to go manual). It's a heavy pedal to make this thing stop. I'd like power assist, but I don't know if 10" will be enough. What do you guys think?

You don't need power assist, you need the correct sized master cylinder. What calipers do you have on the car? More than likely you need a 15/16" master. That will lighten up your pedal a good bit.

I agree with JLMounce. I initially had a 15/16" bore when I switched to manual discs. That master started leaking. Went with a 7/8" replacement. The pedal was noticeably easier from that little change. I can imagine that your pedal effort is very high with 1.125" Also, on the brake pedal arm, you have to go to the hole closer to the pivot. Verify that as well.

I have OEM style front calipers but plan to replace them soon. Rears are "S-10 style"
My thought process is this: If I can make what I have work with a $250 booster, then I would rather do that then have $700 in a new master and booster.

Not sure if 10" is enough. Seems that 12" was more the norm.

You can do a 1" master with those calipers, but I still think I would prefer a 15/16" master on this. You'll have a bit more pedal travel, but I don't think you're going to notice that, or care given the reduction in leg force that will be needed.

Be careful changing the front calipers only and leaving the metric calipers in place. You may find that you create a pretty large bias imbalance as most of the aftermarket stuff from Wilwood and Baer will have smaller piston area. If you are planning on doing new calipers as you state, I'd probably go with the 15/16" MC instead of the 1".

If the car is a street car, or drag car, a 7/8" master can also be used. The pedal will get kind of long, but it'll be much easier on the leg. For things like auto-x where there's a lot of braking, the 7/8" is problematic just because of the amount of pedal travel you end up having to overcome. You're better with a stiffer pedal and shorter pedal travel.

I've run 12 to 13 inches in a street strip car before. With a large booster I had good brakes but if I braked hard several times in a row each time the brake assist became less effective. Also when the weather was cold and the engine was on the high idle cam brakes were definitely harder. Eventually we got a cam ground with the same basic specs but with shorter opening and closing ramps. This got the vacuum up to 14 to 15 inches and the problem disappeared.

At 10 inches most folks look to hydro-boost if they want power assist

I could run 10" with the 79-81 4WD dual diaphragm booster. 9" was a little iffy.

I thought most folks recommended a 1” master for manual disc brakes or is that mostly for manual disc/drum cars?