THSCC Forums

Just wondering about some of the basics:

1) How much vacuum should they see (ball park, I've seen numbers around 10-18" of vacuum, but that is a big range).

2) Size: Is a bigger booster just a bigger vacuum reservoir, or is there more at play (I'd assume a "deeper" booster could contribute a bit more to pedal travel - in a good way).



Why am I asking?

The old Celica now has different front brakes (larger diameter rotor, and larger caliper piston area) and different rear brakes (disks rather than drum), yet still has the original master cylinder and booster.

As it is now, I have reasonable pedal travel, but still have to push awfully hard to get the brakes to do their job (it takes excessive effort to lock them at all). I'd like to get more assist out of the booster, but need to know how to do it.

My first order of business is going to be to add a vacuum gauge into the booster line, just to see what sort of level I'm getting (my engine may not be making sufficient vacuum), but want to learn a little more so I can start to formulate the next part of a plan.

Scott

Ditto! I have plenty of pedal, but my braking system know requires a little more leg effort than before. I have ABS, and can engage it, so my situation isn't a bad a Scott's. Still I'd like a little more "boost" as well.

Maybe I need to see if a ZR-1 brake booster will fit the old Cavalier?

_________________
Vincent Keene
'06 Ford Mustang GT (track rat)
'15 Dodge Charger R/T (yeah, it's got a HEMI!)
'07 Ford Fusion SE (205,000 miles and counting)
'98 Chevy Z-24 (retired)
'93 Acura Integra (Team SWB 24HOL Car)

i don't know the answer to any of your questions, but i do know not to hook up a garden hose to the brake booster vaccum line

_________________
05 MCS

Oh, Oh....Chris, Please tell us how you know that?
Charlie G

_________________
1998 BMW Z3
1987 BMW 325is
2000 BMW
1996 F250 Turbo Diesel

My old 510 had no booster. With Datsun Comp pads and shoes it took about 600 lbs of pedal effort to lock the front brakes. Scott you need more time on the leg press machine.

_________________
George Bright Member # 141
One of the youngest of the old farts.
Yes I voted against you joining the club.

Scott,

I don't know the answers "for sure" here are some "reasonable theories".

1) Vacuum "needs" are probably what is "normal" for the engine and cam(s) that came with the car/booster. In the old days a side effect of more cam overlap was reduced vacuum for the power brakes.

2) I assume (and will look at some books if needed to see if it is covered) is that booster DIAMETER for a given amount of vacuum negative pressure effects available boost (negative psi per square inch times the number of square inches kind of thing). Cannister "depth" compared to diameter would probably be the "reserve capacity" for maintaining boost during low vacuum situations. Fortunately engines tend to have high vacuum when we are braking hard due to the throttle being closed.

3) Except for the old British remote booster systems, most boosters are "in series" with the pedal and master cylinder I think so they would have no impact on pedal travel.

Since you have changed the brake system components a bunch you probably should pick up one of the books on the subject. It is all leverage and displacement and rotor "moment arm" stuff . . . not to mention pad material cf . . .

Dick (heading to his "brake book collection" to see what I got wrong)

_________________
Dick Rasmussen

FS 50 2018 Mustang GT

Adding to my initial comments: According to one of my "brake books" the way most, if not all, vacuum boosters work is that vacuum is supplied to both sides of the diaphragm until the pedal is pushed. Pedal shaft movement opens a control valve which bleeds atmospheric pressure air into one side of the diaphragm. This results in a force towards the MC supplementing the pedal effort.

_________________
Dick Rasmussen

FS 50 2018 Mustang GT

i prefer not to put it in writing...hehehe...ask me at the t&t

_________________
05 MCS

That's how you "flush" the brake fluid Charlie. Geez, don't you engineers know anything?

_________________
Vincent Keene
'06 Ford Mustang GT (track rat)
'15 Dodge Charger R/T (yeah, it's got a HEMI!)
'07 Ford Fusion SE (205,000 miles and counting)
'98 Chevy Z-24 (retired)
'93 Acura Integra (Team SWB 24HOL Car)

FYI,

By the way, don't let gasoline get to the diaphragm (at least British ones). It might soften the rubber such that a new diaphragm is destroyed in a few days. To solve this problem make sure the check valve between the vacuum source and the booster is functional.

_________________
Dick Rasmussen

FS 50 2018 Mustang GT

Scott, back to the mod's you've made:
You have increased the wheel cylinder piston area by a certain amount. Theoretically, you now have a somewhat larger ratio of master cylinder piston area-to-total wheel cylinder area. That SHOULD result in LESS pedal effort required to create the same amount of clamping force (all other things being equal).

Having said that, you must now integrate the larger swept area of the new front brake pads, which will negate some of the advantage, and the possible higher pressure requirements of the rear disc brakes relative to those of the old drum brakes, which operate with a mechanical advantage built in to the design (note that the drum brake wheel cylinders are very small).

When you wash it all out, the booster/master cylinder set-up provided with the car as it was originally delivered (with drum brakes) may be "underpowered" when rear disc brakes are added to the equation.

Maybe check a later model and compare the size to yours. There may be a difference.

'Course, you may have already thought of this...

_________________
Brad