Braking power not what it should be. Is a hybroboost the answer?

[Zedzag]

I have a 1981 Z28 with Baer Track kit 13" front (C4 vette) and 12" rear PBR brakes. I'm using the Baer supplied 15/16 master and an adjustable proportioning valve with the factory 1981 power booster. I've bench bled the master 3 times and the rest of the system and I have moderate braking power. The pedal is firm and not spongy but it feels as if there is little squeeze pressure at the calipers, particularily the fronts. Playing with the prop valve and the rears respond as they should and will lock much easier that the fronts. I'd say the rears are seeing decent pressure at the calipers compared to the fronts.
I have a cammed LQ4 swapped in the car and the cam is fairly choppy. I recently checked engine vacuum with the car warmed up in gear and only get 12" of vacuum. Some people have told me to get a vacuum canister but I think all that will do is give me an extra stored supply of low vacuum and won't increase braking force. I'm considering a hydroboost system but before I doll out the cash would like to be sure first.

Would a different size master help me at all? My pedal seems to get hard a little on the low side of travel. Maybe a larger bore size would build the required pressure earlier in the pedal travel although I'm sure the Baer master is correct and many have success with it.

[72BBSwinger]

That size master is ideal for manual brakes, in the Mopar circles anyway, and is what I use on my car. Power setups usually work best with a 1 1/32".

[Red67Mustang]

Were the front brakes, rear brakes and master provided by Baer as a matched set or pieced together by you? I ask because it sounds like you might have issues with the bore/piston area ratios between the front calipers and master. That could explain the lack of front brake power for a given pedal effort.

Also, i suppose its possible that your MC is bad, bleeding pressure from the front, past the piston seals to the rear.

Going to a larger bore MC will reduce the effective force (psi) you will be able to develop at the calipers (for the same brake pedal force) compared to what you have now.

[Apogee]

If the pedal sinks under steady pressure, then yes, your MC is bypassing fluid and needs to be replaced, however I didn't read anything that would indicate that in the original post. The late 88-96 C4 Corvettes also used 15/16" bore MC's with vacuum assisted brakes, so your bore size is fine IMO...now if you want to go larger, you certainly can, however it's only going to decrease the caliper pressures for a given pedal effort (the Mopar comments don't apply since the caliper piston areas are not equivalent).

First, I would suggest that you check your booster for proper function. Warm up the engine and then shut it off, pump the brake pedal a few times, apply the brakes firmly and restart the engine. The pedal should drop noticeably as the vacuum assist kicks in. If it doesn't, it could be an issue with the booster, the vacuum hoses supplying it, or the low engine vacuum.

An accumulator could help with pedal inconsistency and should store vacuum from all operating conditions, so even though your engine only makes ~12 inches Hg at idle, the worst case scenario, it should make substantially more coming off partial to WOT when you release the throttle. That said, you are correct that it in and of itself does not increase vacuum levels, only an auxiliary vacuum pump will do that.

One other thing to consider is your front brake pads. If you're getting adequate caliper pressures, then your pad coefficient of friction may be too low. What pad compound are you running? Do you have access to a caliper pressure gauge?

Tobin
KORE3

[Zedzag]

The master is correct for the application. It comes with the front system directly from Baer. The rear system is all from Baer as well. The master is not leaking, it's brand new.

Tobin - I'll check the booster function this weekend as instructed when I get a chance to work on the car. The pedal does not sink under pressure, it stays firm but I thought the height was a little low when comparing to other vehicles at our house. The booster is probably original to the car. I previously suspected the brake pads may be the problem and replaced the BAER supplied ones with a decent Raybestos pad for troubleshooting purposes, and the brakes are the same. I don't have access to a caliper pressure gauge at this time.

I'll report back this weekend. Thanks for the ideas.

[Zedzag]

Drove the car for awhile today. Once warm I shut off the car and pumped the brakes a few times until the pedal was hard. I started the car and the pedal did sink a little bit maybe 1". I believe the booster is fine. The car will lock the front brakes from 30 mph or so with hard braking but at 80 mph it takes a while to slow with my foot hard on the pedal. Again, there was no mushy feeling at any time during these tests and I'm confident there isn't air in the system.

[Zedzag]

I'm starting to think hydroboost may be the fix as there is another post about c4 brakes not stopping well.

[Tom Welch]

just to clarify, Tobin is the real 'master' of brakes, call him. You will get a lot of good information.