92 corolla : weak brakes ?

Hi everyone!

I recently changed the pads+rotors on my Corolla because the brakes were not stopping much. The discs were used in an unusual manner: I could see some rusty circular lines on the discs, like if the pads didn't reach the discs, and another side was glazed. Now that I've put about 1500km on the new discs+pads, the brakes don't seem to stop more that before. I can't get the car to stop in a short distance. We also bled the brakes + cleaned the racks and lubes the sliding pins. The technician put grease on the pads+ discs by distraction but I didn't mention it, not sure that it would affect the brakes. But after reading a bit on that, I cleaned everything with isopopilic alcohol + sandpaper the brake pads on a flat surface. I don't lose any oil anywhere and the lines are 1 year old. The calipers looked pretty rusty but didn't seem seized.

It seems that when I use the car after 3-4 days not working, the first "brakings" are powerful and then it dissapears. Weird eh ?

Anyone has a possible explanation on that case? Thanks a lot!

Martin

Okay, let's see where to start....

Brake Pads. If they have had ANY oil, brake fluid, or grease dripped, smeared, or otherwise touched the pad material they should be changed. The brake pad material is porous so anything that is liquid will permeate into the pad and stay there. Also, if the pads have been wearing unevenly, then you should suspect problems with the caliper (below).

Brake rotors. The rotors should be inspected at each pad change and periodically. The rotor wear should be almost nil, with no grooves or tracks worn into the surface. If so, they HAVE to be either replaced or turned (they machine a fresh, flat surface if there is thick enough material within specs). There may be a thin ring of rust on the outer edge where the brake pad doesn't reach...this is normal. If the brake pad is hanging off the edge of the rotor, it is misaligned...this is NOT normal. You can reuse existing rotors but it is advisable to scuff the surface first. I like to use a 3M rust and paint removal disc drill attachment. Run it all around the disc, inner and outer surfaces, to remove the shiny surface. I prefer to make a cross-hatch pattern to be sure you get it all. Doing this will remove any residual pad material that has embedded itself in the disc as well as roughen up the surface so that it can break-in with the new pads.

Brake calipers. The calipers should be inspected for binding if the brake pads show uneven wear or the car pulls to one side when braking. If the pads wear unevenly at an angle across the pad thickness, then the caliper piston is sticking and becoming cock-eyed in the cylinder bore. Some may try to repair the bore, but it is often easier, faster, and cheaper to just buy a new caliper assembly. If the pads are wearing unevenly between the inner and outer pads then the problem is likely the sliding pins that the caliper is mounted to are worn or sticking and need to be greased or replaced. The pins should slide out of the holes in the caliper fairly easily. Wipe them clean of grease and inspect for wear patterns. Any rust or wear spots are cause for replacement. Apply new grease liberally and reinsert. THIS IS THE ONLY PLACE GREASE SHOULD BE PRESENT for front disc brakes. Double check the sliding action for smoothness...it should be smooth.

A special note about the Toyota slide pins: these pins may have a rubber ring around the end of them to help keep them centered in the slide bore. Over time (especially if they are the original parts) these rubber pieces can absorb grease and swell up in the bore, causing binding. Replacement slide pins from parts stores may not have this rubber part and you may be inclined to reuse the existing ones...DON'T. Purchase new slide pins from the dealer. They aren't that much more expensive.

I'm sure I've forgotten some things, but here's a good start. Hope this helps!

Hi!

Thanks for the help!

When I brought the car back from the garage, I uninstalled the brake pads, sanded quite a bit of surface to remove the grease, then washed everything carefully with isopropilic alcohol until I coulnd't see any dirt on my towel. I lightly sanded the discs with 400 grit sandpaper and cleaned them too.

The pins have some wear on them (I can see a little dent in one of them) but the calipers can slide on them. They're a bit hard to slide, but they slide (I put slicone based brake grease).

After I wrote the first message I went for a test drive and inspected everything. The discs + pads seem to use evenly since the surfaces on the discs seem perfect, clean and free of any dirt.

When I start the car and I leave the parking lot, the brakes work great for 2-3 stops. They grip really well and the wheels can lock. Then, it seems that all the power goes away and I can't lock the wheels, neither stop the car on a reasonable distance. They still have some power, but maybe 25%-30% of the regular power...

I still don't understand what is going on.

Thanks again if you can find more possibilities,

Martin

After reading your posts, I'm wondering if the disc brakes are not the problem but perhaps a faulty master cylinder or booster.

What's happening with the pedal travel when you apply the brakes?

Thanks for all your answers! The mechanic where I usually go didn't find the problem and I don't feel like letting him check everything for 3 hours and not finding anything again. I'll defnately go somewhere else, but if I can have a better idea of the problem, maybe it wil be easier to find.


The pedal feels consistent everytime I press on it and I can not press hard enough to bottom out the pedal. It does feel like it gets firmer at the end of the travel tho. And I did not lose any brake fluid since the lines were changed 40 000km ago. There is a definitive lack of power after the 1st-2nd time I hit the brakes everytime I use the car.

Judging by the old discs that were changed, the inner pad on one side did not touch the disc enough to get full power. It seems better now with the new discs+pads : the pads have full contact with the discs, but I still don't have enough power to stop safely.

I though about 3 things :
1. Maybe they're badly bled and the air goes up to the master cylinder when I park the car for 2 days. Then, when I take the car after 1-2 days, it stops well for 1 or 2 times and I lose much of the power. Maybe some air goes into the system and then power goes away ?

2. Maybe the calipers are stuck in rust (brakes were new before it was stored for 8 years) but the still worked flawlessly for 35 000km since. When I cleaned all the parts + caliper rack, I saw some rust around the caliper seals, but the mechanic said that they didn't feel like they were stuck. And that would'nt explain why it stops great at the 1st / 2nd apply of the brakes when I start the car.

3. Maybe it's a booster or master cylinder problem but I don't know anything about those parts. Visually, there is no leak of oil on those other parts, but it could be a bad seal inside the master cylinder.

I'm pretty familiar with hydraulic bicycle disc brakes since I do all the servicing myself. That's pretty much the same system, but smaller. I never had a master cyl. problem on those so I don't know how it feels when they don't work properly.

Thanks again everyone, have a good one!

I recently had to replace my master cylinder because the original one was bypassing internally. The pedal would slowly travel all of the way to the floor, and this doesn't seem to be the problem that you are experiencing. I'm thinking that you might have a problem with your booster, which works off of vacuum, or a vacuum problem. Your owners manual contains a procedure for testing the performance of the power booster. I'd suggest running through that procedure and go from there.

Now I'd suspect old brake fluid, old brake lines, and air in the system. If the pedal is mushy but firm after they contact, there's air. If it's mushy and sinks to the floor gradually, you have a leaking brake booster.

A test for the brake booster is to start the engine:

Vacuum Leak Test

Operate the engine at idle without touching the brake pedal for at least one minute.

Turn off the engine and wait one minute.
Test for the presence of assist vacuum by depressing the brake pedal and releasing it several times. If vacuum is present in the system, light application will produce less and less pedal travel. If there is no vacuum, air is leaking into the system.
System Operation Test

With the engine OFF, pump the brake pedal until the supply vacuum is entirely gone.
Put light, steady pressure on the brake pedal.
Start the engine and let it idle. If the system is operating correctly, the brake pedal should fall toward the floor if the constant pressure is maintained.
Hope this helps!