CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Brake fluid leak when changing shoes?

I replaced the brake shoes on my 99 Altima for the first time yesterday. Never done this job before. It went well except on the driver side, when I was pushing the piston back in to insert the new shoe, a little brake fluid leaked out of the seal on the wheel cylinder. Is this normal? I plan to check the brake fluid level over the next few days to see if it goes down. I’m worried I could have a dangerous leak.

Is this normal?
No

CSA

Did you open the bleeder screw and allow brake fluid to escape through there instead of pushing brake fluid upstream? Was the brake fluid resevoir cap on tight, trapping pressure within the system?

No, I didn’t remove a bleeder screw or the reservoir cap. Guess I deserve a dope slap for that. After I saw the brake fluid leak out I went and removed the cap. It didn’t seem that tight but it could have been sealed I suppose.

The brake fluid cap breaths so that really is not an issue. Chances are as you moved stuff around, you pushed the seal outward a little too far, or the spring inside the wheel cylinder did it for you. The important thing is that after you replaced the shoes and got everything back together, it doesn’t leak.

You don’t want to REMOVE the bleeder screw, you just want to back it out a little bit, allowing brake fluid to escape (preferably through a hose that you’ve hooked up.)

Um, I think what we have is a failure to communicate. First are you talking disc brake pads or drum brake shoes?? Me thinks you are replacing the front pads on the disc brakes. The bleeder screw that we are talking about is on the brake caliper where the pads are installed, not on the resevoir. The idea is not to allow old fluid to be pushed back up the brake line but to exit at the bleeder screw directly at the caliper. At this point the seal on the caliper is leaking. You can make sure the boot is properly in place but its anybody’s guess whether or not it will continue to leak. If it does, it will require changing the caliper itself and the brake pads again, and bleeding the brakes.

Me thinks he was replacing the shoes on the rear.

No brake fluid should leak from a wheel cylinder when the brake shoes are installed. If some does that wheel cylinder requires replacement.

Tester

I changed the drum brake shoes on the rear. At this point I’m wondering if I should just monitor the fluid level in the reservoir and if it doesn’t go down, figure everything is ok; or take it to a mechanic right now and see if I need a new wheel cylinder. Or maybe take the drum off again and check for signs of further leaking?

When you replace the shoes on drum brakes, the new shoes force the brake cylinder piston seals to move further into the cylinder. If there is any corrosion or pitting in the wheel cylinder, the seals will be resting on the bad spot and will leak.

If you are feeling lucky check under the dust boots after a few miles and see if there is more leaked fluid. You may be lucky and the seals may still be sealing. You don’t want to let fluid leak out onto the shoes and drums as the shoes will have to be replaced again.

Follow Testers recommendation and replace the cylinder(s)

I would do the last. I have had this happen to me, you are pulling the springs on for one shoe and it pulls the other brake shoe in too far. This causes the insides of the wheel cylinder to go too far to one side and one seal slips past the end of the bore of the wheel cylinder. A little fluid leaks out but when you get the spring on the second shoe, you push every thing back in place inside the wheel cylinder.

When that happens, you usually get some air into the wheel cylinder so you need to bleed the brakes afterward. You should flush out all the old fluid anyway.

It would not hurt to open the wheel back up and inspect. You can’t be too careful when it comes to brakes.

Sounds good. I did not know that brake fluid would ruin the shoes. I did get some grease on the shoe pads while I was installing them. I wiped it off as best I could and figured that after a little breaking it would be rubbed off by the drum. Is this wrong? Should I replace the shoes again?

The final step in the brake show replacement is to spray the shoes and drum friction surfaces with a brake cleaner, allowing the excess to drip off the shoes, and wiping the shoes and drum with a paper shop towel. The liquid cleaner is is available in a spray can from your local auto parts store. When you take the drums off to check the patency of the cylinder(s), you might just wash the shoes and drums with the spray, wipe with a clean paper towel, and see if the brake(s) operate satisfactorily down the road.

I wouldn’t go changing the shoes again unless I was replacing the wheel cylinder(s) as you may end up doing this job three times.

As previously mentioned, flushing and bleeding the brake system at the time of renewal is good maintenance for the brake system.

Hope this helps.

It’s hard to replace brake shoes without getting a little grease on them. They should be cleaned with a solvent.

Ok, I will open up the drums in a week or two, clean the shoes, and look for signs of leaks. Thanks everyone!

From my experience in replacing drum brakes, the little bit of fluid you lost is the least of your problems. (Some of the comments above provided some rationale for why some fluid was lost.) The bigger problem is that while the shoes were wearing, the pistons within the wheel cylinders moved out in the bore slightly to compensate for that wear. With new shoes, you’ve now pushed the pistons back to where they sat originally when the car was new and the shoes were new. Given that brake fluid is pretty corrosive, you now have a good chance of having the surfaces corrode inside the wheel cylinder causing the pistons to freeze up. The result will be that the wheel’s brakes will fail to operate properly after a period of time when the corrosion sets in. There are two rather simple solutions, but both, unfortunately, require redoing the brake job. The cheapest solution is to purchase a wheel cylinder rebuilding kit (new seals and internal spring) and a wheel cylinder honing tool (which fits a standard electric drill). You remove the wheel cylinder from the car, take it apart, wipe out the old fluid, mount it to a bench vice, and run the honing tool through the cylinder to smooth out the bore and clean it up. Then rebuild the wheel cylinder with the original pistons and the parts from the rebuilding kit. Neither the kit nor the honing tool are very expernsive. A simplier solution is just to buy new wheel cylinders. They cost more than the rebuilding kits, but are not all that expensive, and this avoids the need to do the work on the cylinder. As noted earlier, to remove the wheel cylinder, you first need to remove the brake shoes, springs, etc. The wheel cylinder itself comes off pretty easily in most cases by removing the hydralic line and two short bolts. Once replaced, and the shoes replaced, just bleed the system to get all of the air out.

I’ll add my 2 cents.

By pushing the piston in without giving the fluid someplace to flow to, you’ve forced the fluid past the seal in the wheel cylinder. If you pushed the piston in by hand using a ball=peen hammer handle or such (a common practice) thqn your seal is definitely shot. You should not be able to do this. If you used a piston compressor or a large C-clamp, it might be possible but not without overforceing the piston. With the exception of the square-cut “O” ring that retracts the piston to free the rotor when the brake pedal is released, hydraulic seals and the cavities they operate in are generally designed such that they compress into the cavities and seal more aggressively with pressure applied. Forcing fluid past them means either the seal is worn or may be damaged.

Checking the fluid level is definitely a good idea. But it won’t detect a potential catastrophic failure, a sudden seal failure caused by damage to a seal. I tend to err on the side of safety, so I’d want to change the cylinder out.

Ok, so I need to replace the cylinder and bleed the system to be safe. Got it.

Replace both wheel cylinders. If one starts leaking the other probably isn’t far behind. That way you only have to bleed the brake system once, and not a second time in a month or two when the other wheel cylinder starts leaking.

Tester