Plasticman

My 87 Vette has always had relatively poor braking (no big surprise): I added this info to a previous thread, but evidently everyone who posted on that thread has left (or is no longer interested), so I will start a new one:

1. 2nd owner, Z-52 auto trans, purchased in 1991 @ 23K miles. Brakes never "touch" by dealer, etc. Now have 105K miles.

2. From the getgo, brakes not up to my standards, with poor distance to stop. Pedal always "good" firm, not spongy. Both my 62 Vette with non-OEM front disc brakes (70's era big Chevy calipers with stock 62 drums in rear) and my Dodge Hemi Magnum stop much better! The 87 ABS has always preformed correctly (hammering ABS when trying to stop on ice slick road - Chicago area, not FL). ABS function is not an issue with this Vette - it does what it should, but not "often" now that we are here in FL.

3. Replaced all pads (rotors in great shape, so no change), no essential change in stopping distance. Several different pads tried over the years. Never wore a pad out, just want to try the next best, greatest pad to see if they might just be the answer - Nope!

4. Also change to the "upgrade" bias spring, although it was identical to the one I removed (coil wire dia., number of coils, etc.), and made no difference.

5. Replaced front rotors and calipers with J55 parts. Braking much improved, but still not "great". Note that some of the J55 braking improvement is due to increased leverage of rotor dia. increase, and some due to 2 piston calipers increasing effecting clamping force. Also "think" that the J55 is a stiffer caliper. Stiffer in the sense that it does not bow outward from clamping force at a lower pressure like the original caliper. Have witnessed clamping force deformation on calipers, which is the limit to how much total force the caliper can exert on a rotor until it starts to deflect. Once deflection begins, you cannot exert any more on the rotor - all you are doing is trying to bend the caliper.

6. I have recently changed rear brake pads (again) to Hawks, and have seen no effective change in braking (fronts are currently Raybestos "premium" pads, but have a set of Hawks that I was going to try next). Have used several different pad sets with the J55 upgrade, including Praise pads. Essentially no real difference was ever seen including the OEM pads that were on the Vette originally. Only the J55 front upgrade made any significant difference. And they still are not as good as I want.

7. Have also witnessed the rear rotors' poor stopping power, when jacked up and supported at the rear spindles, auto trans in gear at idle, it takes a great amount of foot pressure to stop the rears from turning. This is not as it should be, and I feel is most of the issue. Simply it is a very small rear brake caliper on a small rotor that cannot even stop the wheels from turning, let alone help stop the vehicle.

My next "test" is to do a pressure check on both front and rear calipers, to verify that the correct brake pressure is reaching the calipers. Have done this before on other vehicles, and it can be enlightening (will tell if ABS is bypass some pressure due to a fault, or ?).

I have read "somewhere" that is was possible to change the rear calipers by installing the original 12" front caliper onto the rear brackets. Have never seen this done, or heard about it again. So I ask if anyone knows if it is possible. With or without a bracket change, adapting a larger caliper to the rears should help increase the rear braking. I understand the limits and reason for forward biasing of brake systems, but rear brakes that cannot even stop the rear wheels from turning is almost biased at 5% at best, when it should be closer to 30%. If I find that the rears are too effective, I can always add an adjustable proportioning valve.

New info: Checked brake pressure at both a front and rear caliper bleeder port (made a fitting to attach a 3000 psi gauge to a modified bleeder). Have done this previously on other vehicles, and always enlightening).

In this case, I applied 100 pounds to the brake pedal (via a standard wgt. scale for consistency) and foot pressure. Both the front and rear calipers read 900 to 1000 psi. From this I deduct that the proportioning valve and ABS system are not at fault. It is simply that the rear calipers are way too small for the intended purpose.

Has anyone upgraded the rear brake calipers on an early C4?

Thanks,
Plasticman

CentralCoaster

The rear calipers and rotors are fine for brake balance, they're almost as big as the front.

GM says you're supposed to have 1250psi/800psi front/rear with 100 lbs pedal force.

Push that pedal harder, see if the rear comes up more. I assume you did it with the car off?

With ABS, there's actually no need to proportion anything. They could simply use the ABS to reduce rear pressure when it was on the verge of locking up. They still use the proportioning valve in case the ABS fails.

CentralCoaster

Whatever you read about early rear brake upgrades is untrue.

The only option I've seen, is a C5 13" caliper/rotor on there which requires modified spindles and loses the parking brake.

There is no bolt on kits that upsize the caliper or rotor.

I was looking at making a 13" rotor kit for the rear that used the 88-96 rear caliper with integral parking brake, but my 85 rear brakes have always grabbed pretty good. Maybe just the ABS sucks. I don't have ABS.

c4cruiser

I use my 87 with the stock diameter rotors for autocross and track days and have no real problems.

I use Hawk HPS for street use and autocross and change to HP+ pads in the front for track use.

What fluid are you using. You might try a DOT4 fluid with a higher boiling point like ATE Super Blue. Doug Rippie Motorsports has a bias spring for the early C4's that will increase braking force to the rear wheels.

Plasticman

As I mentioned previously, the rear brakes can hardly stop the rear wheels from turning at idle! This is with a multitude of different pads, including the current Hawk HPS.

Since I now have the J55 front brakes, a change to the rears would seem necessary for balance, beside the fact that the rears (as mentioned above) are not very effective. As far as being the same size as the fronts, no friggin way. The 87 rears are a much smaller piston compared to the original front calipers.

The above pressure test was done with engine on, at idle. I did add further foot pressure and saw 1100 psi at the rears, with about 115 pounds at the pedal. What I was proving was the pressure to the to the rears was equal to the front, and therefore there was not an issue with either the proportioning valve or ABS system bleeding off pressure that was causing the poor rear brake performance.

The proportioning valve spring was changed to the "hi-po" version, but made no difference, and was identical to what came out (in spring coil wire dia., number of coils, etc.). In other words, t was the same spring GM put in there to begin with.

Been using Castrol LMA DOT4 fluid for the last 20 years in many vehicles, which is flushed every other year (never even gets yellow, let alone dirty).

Plasticman

CentralCoaster

The "hi po" spring should have been shorter and stiffer than the one you removed. Number of coils doesn't mean anything by itself. If it was in fact exactly the same length and same wire diameter, a previous owner probably put one in. The J55 will upset your brake bias towards the front though. You probably lost braking ability by installing it. Right now I'm running a 6-piston 13.1" aftermarket setup in front and it also had no rear brakes afterwards. I added better pads and put in the stiffest possible bias spring I could get (3x stiffer than stock) to keep the rear pressure from reducing. My braking now is better than factory.

I have enough brake calculations at home to make you nauseous. The rear brake piston sizing is perfectly fine. It has a slightly bigger torque arm than fronts due to the shorter reach of the caliper & pad over the rotor. The front piston area is approx 50% larger. The brake bias at 1G decel is approx 35% rear, 65% front, so that 50% piston difference is fine. With that you'd still have to reduce rear line pressure at max braking. But GM reduces it too much. It's the only problem with the 84-87 design other than it's low heat capacity. I don't have all the numbers in front of me as I'm at work. I also don't know squat about the ABS system.

In fact your rear pressure sounds way higher than it should be.

I guess my point is, my 85 brakes were exactly the same as yours other than the bias spring and the ABS, and they would pull the car down from 60mph in 130 feet. So focus on those differences. I have brake pressure guages and fittings but I haven't hooked it up yet.

95% of the complaints you see on here are 88-96 cars. Most are happy with the 84-87 stopping ability, so what you have might not be a common issue like you think it is.

Plasticman

CentralCoaster,

I don't think you have read (or understood) my posts. You are so far out to lunch that I won't begin to answer any further, other than say: J55 probably decreased braking ability???????????????????????????

Please don't bother to answer. Rather hear from someone who knows what he is talking about.

Thanks anyway,
Plasticman

Plasticman

Additional info:

Took the 87 for a 1/2 hour drive around local area, just to heat up the brakes and record temps afterwards. Stopped a total of 18 times from 30-35 mph over this period (moderate braking).

Afterwards, the brake rotors were checked for temp.

Front rotors measured 221 and 235 deg. F.

Rear rotors measure 135 and 141 deg. F.

Ambient temp was 78 deg. F.

Obviously, the rears are not doing a whole lot. Understand the fronts should do about 60% (some "authorities" say 70%) of the work.

Plasticman

CentralCoaster

My bad, I thought YOU were asking the questions.

Enjoy your thread.