Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Compressed brake piston slowly uncompressed by itself

I just completed a front brake job on a 2007 Dodge Grand Caravan, installing new rotors and pads. Right front rotor and pad installation went perfectly. Left front - I heard a faint “hiss” as I compressed the caliper piston using a c clamp. In addition, as I placed the new inner pad against the caliper/piston, I noticed that the piston had become slightly “uncompressed,” meaning it moved back out a little bit on its own. I compressed it again with a c clamp and had to rush to get the caliper back in place with the new pads in place before the piston SLOWLY worked itself back out and I wouldn’t be able to fit the caliper around the new rotors and pads. I completed the brake job, pumped the brakes, and took it for a ride - all seems fine. The pedal is not spongy and it does not travel all the way to the floor.

Do I have anything to worry about? This is my 6th brake job in total. This is my first brake job on this vehicle. The slight hiss and the fact that the caliper piston did not stay seated after I compressed it bothers me.

Thanks for your help.


Well, you shouldn’t be using a c-clamp. Sounds like you’re pushing the fluid back with the c-clamp. Maybe the reservoir is full and you’re building up pressure there that’s pushing back. Whatever the case, you should open the bleeder and compress the caliper piston removing fluid vs pushing it back. Even more so if you have anti-lock brakes. Some of the brake parts, seals and anti-lock, can be sensitive to reverse flow. Also a good thing to change out the fluid completely when you do a brake job.

thanks rripstop; I just opened the cap to brake fluid reservoir - I opened it slowly and heard the same noise (hissing) that I had heard while performing the brake job. There was air trapped in the reservoir. I assume that after compressing the second piston there was too much pressure pumping back ino the reservoir and it began to slowly leak out through the reservoir cap. That would probably also explain why the piston was slowly coming back out after I had compressed it.

I have never used a bleed screw during a brake job. I know that people do use it but I know nothing about how to properly bleed a brake system - so I am somewhat afraid to try that myself. In addition, the Haynes manual that I purchased for that vehicle only suggests removing 2/3 of the brake fluid from the reservoir before completing the brake job. It does not mention the utilization of anything more complex like a bleed screw.

Thanks for your input. What is your opinion regarding the difficulty of a newbie bleeding a brake system?

Do you have anti-lock brakes?


As I understand it, on an ABS system its best to always use the bleeder to expel fluid in order to compress the caliper. The ABS systems can be under high pressure & those very expensive ABS units don’t like being subjected to what amounts to a pressured back flush. I stand open to correction on that but I have always used the bleeder with an ABS system. Its not a problem re: air in the lines if you don’t let the master cylinder run dry and if you close the bleeder after getting the piston in.

Thanks cig - I have also heard that people do complete the process that you outlined. I have never done that for basically 2 reasons: one - I don’t know how to do it properly, and two, the Haynes manual that I purchased for the car does not list that process when completing a brake job. I therefore decided to trust the manual.

I stopped trusting Haynes a long time ago. I’ll bet you that about half the text in that manual was first written for something like the 1983 Caravan. I actually have a manual for a '97 Escort and I know for a fact that a bunch of the info in it wasn’t updated for the newer generation Escorts.

Anyway, Autozone has free online repair info if you enter an email address. I will often compare my Haynes to that & sometimes use the internet to even double check all of that.

It would be best if you have a friend that has done this before to help you, you need a second person anyway. You will need the procedures for the antilock system, on some, you just remove the plug and bleed them the normal way.

In the future, with antilock brakes, you should always open the bleed valve when compressing the piston back into the caliper. I don’t see anything wrong with using a c-clamp, I use a large pair of channel lock pliers myself. It looks like you didn’t do any damage this time, but the antilock brakes do not like large amounts of reverse flow in the system.

After any brake job, you should flush the system. It is preferable that you don’t get any air into the system while doing this. even though it is best done with two people, I recommend that you get a one man bleed kit. They only run about $5.

You will need to put all four corners of the van on jackstands and remove the wheels.

Use a bulb, like a Turkey baster to remove most of the brake fluid in the reservoir. The fill with fresh brake fluid. Start at the passenger side rear wheel. First put a box end wrench on the bleed screw, then slip the hose of the one man bleed kit over the nipple of the bleed screw. Hang the bottle above the bleed screw.

Now timing is critical so make sure that you and your helper have a good communication system worked out, and I don’t mean walkie talkies. I mean that the one at the brake gives the command and the one in the drivers seat responds when each command it completed.

Lets call the two positions brakeman and driver.

Brakeman calls "press"
Driver steps on the brake pedal and responds pressure
Brakeman opens bleed screw, driver responds “down” when the pedal goes to the floor
Brakeman closes bleed valve and calls "up"
Driver lifts foot off brake pedal and responds “up” when the brake pedal has returned to its up position.
Repeat until brake fluid starts coming out clean.
Repeat for each wheel, rears first, then fronts.
Every time the bleed kit’s cup gets full, call “hold”. empty the cup, check and add brake fluid to the MC reservoir.

thanks - I’ll take that into consideration next time.

I have a 2002 Chrysler minivan with anti-lock brakes. The Haynes manual warn not to attempt to bleed the anti-lock brakes and tells you that if you need the brakes bled to have it towed to the dealer because you have to have computer codes input to hold the ABS valves open so the system can be bled.

thanks oldtimer. I have no current need to bleed my brakes. What I am confused about is this: A (Haynes) repair manual for my Dodge Grand Caravan does not mention anything about using a bleed screw in the process it outlines to replace the brake pads. I followed the process outlined in that manual to replace the brakes and I think I have completed it successfully (meaning it appears that the new brakes are working as they should).

I have used this same process to successfully replace brakes on a 2008 Pontiac G6 (with ABS) and a 1998 Pontiac Grand Prix (with ABS), never using a bleed screw and always using a c clamp to compress the piston into its bore. By the way, this process that I have used successfully was taught to me while I was in high school (20 yrs ago) by my uncle, a 35 yr Cadillac mechanic - now retired.

I agree that ABS and other systems have changed since the time I was taught the process. I even checked with my uncle within the last year regarding the c clamp process - he claims I can still use it. Now he is up in age and I am willing to say that he may be too far removed from the trade to now provide me with up to date info.

So - In conclusion, all I want is to know how to complete a brake job correctly, without the need for potentially expensive repairs later. I don’t know who I can trust. I purchased a manual for my specific vehicles (Dodge and Pontiac) and followed that process and so far no problems. If the c clamp was not supposed to be used any longer because of ABS why is it in the manual? I’m not nieve, people and manuals are not perfect - but I thought that I could trust it. In addition, I wish I could talk to a dealer mechanic and ask him/her how they compress the piston…

Thanks to everyone for their thoughts. I am just a guy who enjoys accomplishing some relatively minor repairs on my own, saving some money for me and my family along the way. But as I said earlier, I just want to learn how to complete the job the correct way.

The factory manual would tell you to open the bleeder screw while you are compressing the piston. Its very easy to do with a tube on the bleeder going into a jar and you won’t cause a problem with ABS or the master cylinder by pushing the old fluid back up the line.

I’ve done enough brake jobs over the years that I strongly feel that opening the bleeder is pretty much relegated to the “old wives tale” category. Sounds like Dingo’s issue was too much fluid in the reservoir. We had an overfilled one, thanks to the local quickie oil change place, that the brakes were actually dragging while driving the car down the road. Sucked out some fluid & it was fine.

I. for one, prefer to use the bleeder. Even if one doesn’t totally purge the fluid with that change, it lets some of it out at the caliper to be replaced with fresh juice.

The way I recommend doing it is simple: Half fill a clear bottle with fresh fluid. Then hang the bottle close to the caliper and run a clear tube from the bleeder into the fluid. Then when one opens the bleeder and compresses the caliper, the fluid pushes the air in the tube into the fluid in the bottle and it burps out. Any backdraw pulls back only fresh fluid. No air enters the caliper. As always, be sure to keep the reservoir full.

It’s easy, safe for the system, and replaces the fluid in the caliper with fresh fluid.

Even with no ABS and no “pee bottle”, the cap should be opened the MC when pushing calipers in. And a turkey baster should be used to draw out aome of the fluid to prevent it from overflowing and making a mess.

There is nothing wrong with using a C-clamp. In fact, because a C-clamp compresses the piston so slowly, the reverse flow of the fluid is slow and maybe why you didn’t hurt the ABS.

I’m not that patient, I grab a large pair of channel locks and squeeze that piston back in in just a few seconds. There is a special caliper compressor that you can buy but it works just like a C-clamp, and is just as slow.

I also use a C clamp when compressing the piston on my cars, but I’ve never changed the brakes on any of my cars that has ABS yet. Since reading this when I do one that has ABS I’ll use the bleeder valves. Bleeding brakes is a very simple process with two people just as described above. When I need to bleed mine I use my wife to depress and release the brake pedal as needed while I open and close the bleeders. If my wife can do it with my instructions anyone can. lol

“I use my wife”…

Me, too. She seems to follow directions very well. It’s a simple process, and I can tell you she’s already really, really good at pressing on pedals (she uses 3, every day, clutch, brake and lots of gas)… :slight_smile:


mountain bike, FordMan, and everyone else - thank you for your input - it is truly appreciated!