brake issues

[sam 74]

i'm haveing some issues with my power brakes, i was thinking about adding a vacuum assist since i have a large duration cam and my vac reading at idle is 7" with about 15-20 at cruise. the car is a 74 Olds Cutlass, it has a factory booster, i believe its 9 inches, it has the stock MC disc/drum. i have and aerospace 4 piston rear kit on the back now and i have the factory calipers up front, going to swap to GMIII calipers soon. i have no plans to swap rims at the moment so 15 inches is it so thats why i'm sticking with stock type calipers a rotors. i have issues with the pedal pretty much being floored to stop, and i'm not comfortable with that so i was thinking about adding vacuum assist to help with the low vacuum issue, so the question is will this help get the pedal off the floor a bit so i get a full sweep on the pedal?

[Apogee]

How thick is your booster, 3.5" or 6.5"? Depending on your Cutlass, it could have come with either a single-11" diaphragm unit or a dual-9" unit IIRC.

You definitely have extremely low vacuum at idle, however low vacuum should result in a high, firm pedal, not one on the floor. A low pedal could indicate air in the system and/or an undersized master cylinder. The original MC bore size should be 1.125" for a power disc/drum application. What have you done with respect to making sure the system is fully bleed?

Assuming you plan to stick with vacuum assist rather than convert to manual or hydroboost, I would suggest that you verify the bore size of your MC, bleed/flush the system to provide good clean fluid and remove any air, then see where you stand. This should get the pedal off the floor, whereas just adding a aux vacuum pump could actually make things worse initially with respect to pedal travel.

Do you know what the effective piston area of your Aerospace calipers is in the rear? The Wilwood GM-III calipers are great way to shed weight, however they will also reduce the effective piston area of the front calipers from 6.7 square inches to 5.94 square inches, an 11% decrease in clamping force, and will reduce the rigidity of the caliper as well.

Tobin
KORE3

[sam 74]

i don't know the effective area of the aerospace pistons. i can measure them the next time i get the rear wheels off, the booster is 6.5 inches thick. i just double checked the MC and it is 1 1/8 inches. when i swapped my brake lines about a year ago i bled the brakes with a vacuum bleeder and it seemed like all the air was out, i can change out the fluid and re bleed the system and see how that turns out, but it seemed like when i swapped the rear discs on the pedal went to the floor. i don't have a set of drums anymore to swap back to to check plus i now have a nine inch that needs a retainer plate for the bearings and my drums were 10 bolt drums with C clip axles from the factory rear that i got rid of. so all in all i'm a b it lost at this point.

[sam 74]

the pistons measure 1.5 inches on the rear calipers.

[Apogee]

the pistons measure 1.5 inches on the rear calipers.

So with 1.5" diameter pistons, that equates to ~3.53 square inches of effective piston area, which is about correct given the OE front calipers. Your MC bore size of 1.125" should be fine, narrowing it down to most likely a bleeding issue. It might be time to isolate the MC by plugging both outlet ports and making sure you can get a good pedal, then add in the rear and front brake circuits one at a time, making sure you can get a good pedal after each addition. There's a problem, you just need to find it.

Tobin
KORE3

[sam 74]

i should clarify a little, i have 1 decent push on the pedal, after that its on the floor, i'll be changing out the fluid as soon as i have some free time and re bleed everything to see if that helps at all, i do have a vac canister as i can not run PB without it. i was hoping adding the extra vacuum from a pump might help out. i can't imagine that there is anymore than the absolute min. vacuum available in the booster to run the brakes. if the new fluid and re bleed do nothing i'll try isolating the MC and see what happens, thanks for the help.

[Apogee]

i should clarify a little, i have 1 decent push on the pedal, after that its on the floor, i'll be changing out the fluid as soon as i have some free time and re bleed everything to see if that helps at all, i do have a vac canister as i can not run PB without it. i was hoping adding the extra vacuum from a pump might help out. i can't imagine that there is anymore than the absolute min. vacuum available in the booster to run the brakes. if the new fluid and re bleed do nothing i'll try isolating the MC and see what happens, thanks for the help.

Again, a low pedal scenario is not what you would expect from a low-vacuum engine. You would expect the pedal to have less boost and therefore feel firmer and be higher than the previous application. Your pedal should never be on the floor unless something isn't bleed properly or typically rear parking brake calipers aren't adjusted properly, but in your case you can rule out the parking brake since you have Aerospace calipers.

Tobin
KORE3

[sam 74]

well last night i had a chance to really use the brakes, i was taking my mom to an urgent care because she has some health issues at the moment, while driving there, yes it was a little spirited, i had a guy cut me off and hit his brakes because he was trying to make the left turn he just missed and it forced me to slam my brakes, while the car stopped ok it didn't even feel like it was close to locking up with the pedal on the ground, i'll re bleed and change the fluid out while i swap out the front calipers. i did notice that when i use my line lock to lock out the rear brakes the pedal becomes firmer.

[Apogee]

Are you sure that your MC is the stock 1.125" bore size?

Tobin
KORE3

[sam 74]

maybe i'm measuring at the wrong point.

EDIT: i just remeasured and its 1 1/16 on the inside piston area.