Hello Folks,

I drive a '92 convertible and I experience intermittent vibration from the front brakes. The car sat for 3 years so I removed the front calipers, compressed the cylinders, and expanded them by depressing the brake pedal. I ensured adequate brake fluid (new) was always in the reservoir. I repeated the above twice and then bled the brakes. I also applied anti-sieze to the pins and glide points. When driving at slow speeds (below 60 or so) I do not notice any problems. Above this speed it feels like someone is hitting the bottom of the car with a hammer. When I apply the brakes it gets worse. If I hit the brakes very hard it sometimes cures the problem. As I mentioned above this condition is intermittent. I can drive the car one time without any problems but will get this condition the next time I drive.

Is it possible one piston within the caliper is closing (without applying the brake pedal) and causing the brake pad to be applied to the rotor in a misaligned position. If this is possible then my question is what would cause this condition? Worn master cylinder?

Any advice is greatly appreciated.

 

Get new rotors and stop pressing the brake pedal with the calipers off the car.

 

 

The hammering could be a wheel bearing problem. Each wheel hub has 2 bearings in it. an inner and an outer. It could be one of them . Or, you screwed something up on reassembly. or, there is a defective rotor on the back side that you are not seeing, or rust on the rotors forced into the pads.
You should not depress the brake pedal with the rotors not installed. you can create leaks, cocked pistons, intrusion of debris, and other maladies by that kind of action.
You can do more damage to the o rings and seals that way, and allow fluid leaks to destoy your pads.
If no leaks yet, you might be lucky.
If you want new fluid, bleed them till it runs clear, making sure that the fluid never goes dry in the master cylinder.
Don't worry that the calipers have sat in one place for a long time. If the seals have taken a set, or if the seals aren't sealing, and you have leaks, you will know about it soon. just pull a caliper off, and run a .020 feeler gauge in where the boot meets the piston.
If you are worried about it, pull the calipers off, and overhaul them.
If no overhaul, open the bleeder, and squeeze the pistons back into the housing, close the bleeder, reinstall the calipers and then pump the pistons back into place, bleed the air out, and move on to the next wheel.
If it sat that long, there is going to be water in your fluid because brake fluid has an affinity for water. Water goes to the lowest point in the system if gravity is allowed to work. The lowest point is your calipers if unhindered. Usually, the master cylinder is where the fluid collects water, so the first pump usually contaminates the fluid downstream of the master cyl.
If it sounds like we are treating you like a moron, buy a book and read up on proper brake maintenance procedures.
I would not want you behind me on the road after you have been messing with your brakes. That is irresponsible.
Do it right, do it right the first time.

 

Folks,

Thanks for the input. As far as depressing the brake pedal while the caliper was off could sound irresponsible and stupid. I placed "stoppers" in the calipers so the extension of the pistion would not exceed a safe distance, i.e. wear of half the pad. Obviously no one could know that if I don't state such. However, I have been wrenching on vehicles for years. I need to remember that not everyone who works on cars is mechanically inclined so some odd stuff gets asked on forums which makes people scratch their heads. :-)

I do have a FSM and just wanted some input from folks who may have experienced a similar condition before I began replacing rotors, calipers, and/or master cylinder.

FYI, I did put new shocks and tires on all four corners plus I replaced the right front wheel hub.

The rotor replacement seems odd only because the condition is not alway there. I will remove the calipers/rotors once again to ensure all surfaces are clear of any debris and once again bleed the system. I've done this before but something seems to be "hanging" during the braking.

Coupeguy2001 - I felt I did the job right the first time. Certainly not my 1st or 20th time I have worked on brakes. Rest assured I am concerned about the safety of others and of my own. The car has not moved since this occured. Even after working on cars so long it is always good to have a second set of eyes check out the situation thus I wrote my thread. Thanks for your recommendations.