front brakes lock up and vibration

[inthedoghouse]

Issue:

• Front right brake locks on hard stops - feels solid and stops straight on regular stops
• Random vibration in steering wheel - cannot seem to find a constant speed/temp that causes it

Recent work:

• new OEM rubber brake lines front and rear
• new rear rotors and Porterfield pads
• front rotors and pads (OEM?) are old but in good shape
• flushed fluid with ATE/power bleeder and manual method
• brake pad bed in - then a 1400 mile road trip
• new turbo tie rods
• alignment

I have noticed with the car in the air, the front right wheel does not spin as long as the left wheel. Could this be wheel bearing adjustment/wear or caliper sticking? Could a sticking caliper or a wheel bearing also cause my steering wheel vibration?

I was considering road force balance also.

Car has 155k on it and I don't have any evidence the wheel bearings have been replaced but they were adjusted at 110k.

What order should I attack this - road force balance, wheel bearings, caliber sticking?

[DRACO A5OG]

whoa, 45K miles since last bearing adjust, that's allot, check them again, I bet they are loose. I check mine every oil change 3K.

Then try to repeat the vibration, then move to balancing the wheels.

Last would be caliper rebuild.

[inthedoghouse]

I finally had a chance to adjust both wheel bearings. One was a little loose and one was a little tight. They both seem pretty smooth now and turn the same while on jack stands. The car tracks straight now under braking but both front brakes are locking up now with medium to high pedal force above 40mph.

What should I try next
1 - more bleeding?
2 - rebuild calipers?
3 - brake pad material is different front to back. Rear are brand new Porterfield and fronts are unknown but have lots of life. Would this really matter this much?
4 - front to back bias vale has air in it?

[zedsn]

Sounds like your car will need both front calipers rebuilt if the car has need sitting awhile. I did this recently and is not that hard to do once you get the pistons out.

[DRACO A5OG]

Try exercising the pistons to insure they all are moving at the same rate.

This would require you to remove the pads and use a piece of wood of half the thickness, pump the brakes until the pistons are out, place a 2x4 on the pedal depressed in about 2"s, go to the caliper, open the bleeder valve and hand push the pistons back, they all should feel the same, If any of them stick, that will most likely mean it is time to rebuild the calipers ( really should be called replace the inner seals and cleaning ).

Another note, rears will take some time to break in to the the rotors so the fronts are working extra duty until that occurs, there is no break in "bedding" period for Porterfields so you just have to drive her until rears catch up to the fronts.

Air in the proportioning valve? I don't think so or you will have soft breaks but look for wetness in that area.

[inthedoghouse]

Actually I drive it fairly often - just returned from a 1400 mile road trip. Before I changed my rear brake pads and rotors and replaced the 4 flex lines, it worked fine and did not lock up. Now the front brakes are locking up. What are the typical symptoms requiring a caliper rebuild?

[DRACO A5OG]

Pulling if a piston sticks, it pulls to the side that is working properly

[kyngfish]

Have you checked the rubber boots around the pistons? If they are broken or cracked I would go ahead and do the rebuild before they lock up to the point where it's a real pain in the ass to get them out. Ask me how I know . Thanks to DRACO and my neighbor with an air compressor I'm well on my way but it wasn't the most fun I've ever had.

[inthedoghouse]

got it...so if both fronts are locking up then seems unlikely both front brakes are sticking at the same time. Pedal is firm but maybe there is a clogged line somewhere. I guess I will try to bleed all 4 wheels again.

[DRACO A5OG]

Yes, it is possible, all four should bleed at same rate to insure there is no clog in the system.

Far as pressure, the proportioning valve will reduce the flow to the rear about to 70% on our 3.2s but bleeding one at time should be equal maybe more flow up front or should be.

[Jdub]

Replacing the rubber lines with new may have had the effect of exposing issues with your front calipers. I agree until the rear brakes "catch up" in braking efficiency your front brakes are taking more than their usual percentage of load.

God I hate to say it and what a PITA but if I were in your shoes with all the good work you have put in I would absolutely rebuild the front calipers or just purchase a rebuilt set. Lots of great information via a Search that will prevent heartbreak.

[Trackrash]

Are you saying that the vibration is gone since you adjusted the front bearings?

Are the tires and front rotors spinning true when you turn them by hand with the front end jacked up?

I would replace the front pads with matching pads to the rear.

[chris_seven]

This is becoming a bit difficult to understand and some of the suggested causes seem confused and I think it needs unravelling.

If the front bearings are worn or badly adjusted they will tend to cause vibration and they will cause the steering to pull due to hub movement and loss of wheel alignment.

I don't think worn or badly adjusted bearings will cause the front brakes to lock.

If the front caliper pistons are sticking then again they will tend to cause the car to pull and they will also reduce the deceleration unless pressed hard which could result in them breaking free and then producing too much braking as the stiction has been overcome.

This symptom would mean that there was an associated increase in the required pedal force and it would be useful to know it this is the case. The brakes would also tend to snatch as the pistons break free.

It is also important to realise that for a given pedal force the braking effort on either the front or the rear will not change if one set of calipers doesn't work.

What does happen when one set of brakes doesn't work is that for a given deceleration pedal force needs to increase.

If the rear brakes are not working correctly then the pedal force needed for a given declaration will mean that the fronts may lock as the rears aren't helping to slow down the car.

Refurbishing the front calipers won't solve this problem.

The 'badly' named rear bias valve only affects the brake balance of the car once it reaches the pre-set pressure.

Until this point the rear brakes will provide a greater proportion of the total braking effort and the influence of this valve is to reduce the tendency of the rear brakes to lock when used hard.

Normal gentle to moderate driving is unlikely to make this valve operate.

Unless the valve has become blocked it will have little influence on bleeding the brakes as without pressure it will not operate the pre-set spring and reduce fluid flow to the rear.

Unless the locking of the fronts is very 'snatchy' and if the pedal force needed to slow the car has increased I would suspect the rear brakes, I would bleed them individually and monitor the fluid flow per pedal stroke and check the pressure limiting valve.

[DanielDudley]

You also need to check your corner balance. But if the front rotors aren't new, you could also have glazing issues that won't always go away with bedding.

[inthedoghouse]

Thanks everyone for the comments and suggestions. I was a bit slow to get back to troubleshooting!

I am making progress and its getting better. This weekend I changed out the front brake pads. The old ones had plenty of material but looked glazed and shiny. I installed some Porterfield pads just like the rear.

Nnow I have all new pads with same material. New rear rotors. Old front rotors. I have done the bed in procedure a few times. I've also adjusted the front wheel bearings. Now the car stops straight and true. The pedal is right at the top and firm. Under light to moderate braking, its working great. Under emergency braking, the fronts still lock - never the rears. Can you glaze a rotor also? Maybe my front rotors are shot?

Based on all the feedback, I think I will bleed the rears some more to see if perhaps they are not getting full pressure. Its certainly driveable now, but I thought if its all working right then brake lock would likely be all wheels vs. just one axle.

[Flojo]

actually you want locking up fronts when emergency braking (as a non-ABS car).
As long as it doenst pull to one side, but brakes straight, then ist fine.

If problem persists: go to a pro, check master cylinder (or replace) and flush system.
(this helped me out of trouble after I changed all 4 calipers, flexlines, rotors and pads; the rear right was clogged after T-valve)

[zedsn]

With lock up in some cases I have found that the caliper hoses are shot and need replaced. The rubber breaks down on the inside. I would replace them next if the problem persists.

[manbridge 74]

Your rear calipers need rebuilding/replacing.

Your car is 33 years old. Recommend replacing rubber line sections too.

[davewadd]

Consider corner-balancing. I had my left-front locking up on hard braking during autocross, and it turned out that the car was set up as if it had a much heavier driver. Without proper weight on one wheel, it would be too light, and lock sooner that the other front wheel.

[Flojo]

Originally Posted by davewadd

Consider corner-balancing. I had my left-front locking up on hard braking during autocross,...

depents which way round the track goes (left- or right orientated) and on how your trunk compartement is stuffed. just envisage all the stuff to the left (driver, battery, tanknozzle, wireing...) compared to the right (oilcooler only, if spare is deleted).
and also if you have a strut brace or not (lesser locking without).

I stay with my guess: main cylinder check and system flush