Frank43

Hey all,

Re-habing a project LT4 (with J55 Brake package). Flushed fluid, bled lines, and wire brushed minor surface rush off smooth thick rotors. Pads look good, but car was sitting for 7 years.
Car is running now and I have a firm (feels good) brake pedal. But, on test drives, car is slow to stop. Fades to a stop.
Is this a Master Cylinder issue, or something else? Never changed pads, can they get old (for lack of a better term)? I got the rotors hot, but brakes just don't bite. Line pressure was fine to hand- vacuum bleed brakes when job was done last week.
Thoughts?
Frank
ps. Curious, do all LT4 cars (1996) have J55 brake package...?

MatthewMiller

That's an odd problem. Have you verified that all four calipers are actually working and gripping the rotors when you press the pedal? I wonder if one of them (especially a front) is frozen/rusted open? If so, you would still get a firm pedal but no braking on that wheel at all.

Also, yes, all LT4s got the J55 brakes.

Frank43

Thanks. Whats the best way to test?

ghoastrider1

seems that its a frozen unit or perhaps the front brakes are nor being engaged smoothly with the rear brakes. Air still in system? Worn cup in the master cly ?

MatthewMiller

You might start by jacking each side of the car up, having someone press the brake pedal, and just seeing if you can turn one of the wheels by hand. Probably an easy way is to use a cheap infrared thermometer gun (~$30 at Autozone or Harbor Freight, among others). Go for a short drive on a big parking lot or deserted road, stop hard a few times, stop and put the parking brake on, and then see if each rotor or caliper are similarly hot. If one of them is still close to ambient or is at least significantly cooler than the others, then you've identified the culprit. Another way might be to take each caliper off and see if a brake pad spreader tool can easily push each piston back in all the way. These are ideas off the top of my head.

I do think that if there were air in the system or wear inside the m/c, you would probably have a soft pedal. But if you find either that both front or both rear calipers aren't functioning, then perhaps the m/c is the culprit and should be replaced.

Frank43

So I pulled the tires/jackstands and the front left is pushing major bubbles through my bleeder. The rest are normal. Think this is a brake line? Or caliper seeping air?
Frank

MatthewMiller

I'm still not convinced you actually had air in there. If you did, your pedal should be very soft and probably go to the floor. It depends on what kind of bleeding equipment you're using. If you're using a vacuum system (e.g. MityVac), then you may just be drawing air around the threads of the bleeder screw. You could try wrapping those thread in Teflon tape to stop that.

ghoastrider1

You get this problem fixed?

Silver85

I would bet you have seals in the caliper that are dry rotted and need to be replaced. You can rebuild them. Kits are typically not expensive. As long as there isn't too much pitting on the piston they are reuseable.

Glad to see you got her on the road! Did you get the rough running issue fixed?

Frank43

I will investigate this weekend when I have time. Car is indeed running, but a little rough still. I need to check on a couple wire connections close to the firewall/pass side as well. I had troubles getting a good fit with the tight workspace.

Time is the biggest challenge! LOL
Frank

gerardvg

I would replace the brake pads with hawk street brake pads, when using a G clamp to depress the piston into the caliper you will know if it's free or seized. Also listen to the booster for any vacuum leaks?

You need a high friction brake pad material on a C4, soft gentle to rotors brake pads will not give the stopping power you need.

I have J55 brakes on my c4, she needs very little brake pedal effort to stop the car quickly. They do not fade on track days, lap after lap of braking with confidence.

aklim

You have ABS? If so, are you sure that air didn't get into the ABS unit? If so, the best way is to bring it to a dealership with a Tech One scanner to actuate the ABS while bleeding.

Klondike

I had a similar problem with my LT4. The rear brakes seemed to not do too much to help out the fronts in slowing the car. If I got on the brakes hard, it would start locking up the fronts and would activate the ABS. Changed pads and was not much better.
Finally changed the spring in the proportioning valve to the DRM heavier spring. Really woke up the rear brakes! Car stops faster and flatter without nosediving or setting off the ABS any more.

aklim

Interesting. How were the rubber lines? Were they decent or old? Did you need the dealership to bleed the brakes after install since they are before the ABS?

Klondike

Changed rear brake lines to the braided steel lines. My Wife got me a brake line kit a couple of Christmas ago. I Put on the rears before I realized that the fronts wouldn't fit the J-55 calipers. They were for the standard C4 brakes and have a different fitting on the caliper from the J-55's, so I still need to get lines for the fronts. Been forgetting that! I bleed my own brakes and have never had a problem doing them. I put the one-man Speed bleeders in all my calipers and it makes it a lot faster. I also do both a front and rear on one side at the same time. It lets the master cylinder do a full stroke without the pressure from the rear limiting the stroke for the front chamber & vice-versa. Just don't bottom out the pedal and check the fluid level after 3 or 4 strokes, It moves a lot of fluid through the lines pretty fast! Never had a problem with air getting in the ABS unit, maybe I'm just lucky with that.

aklim

IDK. I have never gone that far before. I have always kept fluid in the reservoir and known of cases where it went empty and the brake never worked right till you cycled the ABS solenoids which is done by a Tech 1 scanner.