76FJ55

I have been trouble shooting the brakes on my 85 Auto, and have a few questions. Well I think I have problems, but I guess the consensus could be that it is completely normal.

As long as I've had the car the brakes felt this way. I new I needed pads and rotors, so figured brake performance would get better after those where replaced.

now for my questions and symptoms.

It seems that my brakes take more effort than I would expect to stop the car. It will definitely lock the fronts if I push hard enough so I believe my problem lies with the rears.

This is the first Auto I've owned so I don't have anything to compare it to. With the car off the ground (on Lift bars - Ken's a genius). How much driving force does it have at idle? It takes a high amount of pedal effort to stop the rears even at idle and with any throttle input I can't hold the rears front turning. With light throttle by the time the revs hit ~1200 RPM I cannot hold the wheels from spinning no matter how hard I push on the brakes. How close ti TC stall should I be able to get with the rear brakes?

How far should the pedal travel to hold the car? with the car at a stop in drive and idle where should the pedal be to hold the car stationary? Mine is dead level with the throttle does this seem correct, or low due to a rear brake channel issue?

Here is what I have done so far:
-New pads and rotors on all 4 corners. All calipers looked good and pistons moved freely
-Bled, bled and bled again. Both from master to farthest caliper and farthest caliper to closest caliper. Car has speed bleeders so I've pumped a lot of fluid through them, and had my wife help and did the old push and hold, open bleeder, close bleed, release method.
-new master cylinder (no noticeable change)
-Then gutted the proportioning valve (no noticeable change) so full brake pressure should be reaching the rear brakes, but on pavement I still can't lock the rears. (I plan to do a detailed write-up on how it functions and controls both the cracking pressure and proportioning ratio, but first I'd like to focus on getting my car dialed)

So I'm looking for any and all input, feedback and suggestions.

Mrmerlin

by gutting the P valve what does this mean??
Usually poor performance is due to a failed MC but since you have replaced this part then the next thing is the Proportioning valve, replace it.

Also if you used a clamp on a flex line then this could be crimped internally.
Then the other thing you can do is inspect the brake lines for a crimped area.
Also do you know that the booster is functioning correctly?? If you try to remove the booster vacuum line does it let air rush in after the engine is shut off ( to test this spray some silicone on the rubber insert so the barb valve can be pulled out)
What fluid are you using?? I suggest that you use ATE super blue fluid

76FJ55

When I state that I gutted the P valve, I removed the internals of the valve so that it is through passage and no longer limits the pressure that goes to the rear wheels. I did this to eliminate the PV as the cause of my rear brakes having little holding power. I thought that if the PV was prematurely limiting my braking pressure that it could be causing my poor brake performance. by removing the internals of the PV and allowing all the pressure generated by the master to reach the rear brakes the rear should presumably lock before the fronts and would verify that the rest of the system was working okay.

I did not clamp any of the flex lined during the process so that shouldn't be an issue. And I have inspected the entire length of the line running from the master back to the rear calipers and everything appears good.

the booster seems to be functioning correctly, as when I pump the brakes up and then start the car the pedal drops slightly as expected.

I am using Valvoline synthetic DOT 4 brake fluid.

76FJ55

does anyone know how much power the rear brakes should hold when the car is lifted off the ground? I am totally perplexed by my lack of rear brake power, but it may be completely normal.

Can I get ATE super blue locally or is it something I will need to order? I got the Valvoline DOT 4 because it is the only DOT 4 at the parts store by the house. I wouldn't think the valvoline should have any effect as it has only been in the system for 3 days.

I can't seem to think of anything left in the system that should be effecting brake power that I haven't already looked at?

Oh yeh, thanks for the Ideas and please keep them coming. I'm sure eventually the solution will be presented.

nsantolick

I'm still trying to understand what's wrong with my car, a '89 S4 auto. The rear brakes, at times, are so poor that when I put it in reverse (on a gravel driveway), I've experienced the car being pulled backwards, despite locked up front brakes, NO MATTER HOW MUCH PRESSURE I APPLY TO THE BRAKE PEDAL...

My car has had stainless brake lines installed, fluid flushed of course, cut rotors and new semi-race pads. I was about to try bleeding the system again, as I suspect that the mechanic who did the job bled my ABS car with the ignition off.

I've heard that with the ABS cars, you must bleed the brake system with the car powered up. Is this true? What would be the consequences of bleeding it with the battery removed?








Nick - 89S4

Mrmerlin

the syn fluids have a tendency to swell the seals over time, this usually damages the MC seals first then calipers start to stick.
I would go with the ATE super blue.
Also you either have air in the lines or the MC has a damaged seal for the rear line